I worked for a while at an LED startup doing photometric testing, and I guess I still like writing about lighting because I just spewed out all this pretty quickly. Specific recommendations are in the last paragraph if you don't want to read it all. Hope some of this helps, and keep up the good work on that Mercedes! It's a very cool build, and exactly the sort of thing I want now that I almost had a factory 190E manual Sportline package (It had some weird title shennanigans going on, which thankfully the current owner managed to sort out).
Regarding the question about brightness: The unit you want to look for is "lumens" or "luminous flux". The lumen is a measurement that tries to compare how bright one light source is relative to another. It gets complicated because, as in the case of LEDs and fluorescents, they put out different amounts of power at different wavelengths, and human eyes have different sensitivities to different wavelengths of light. The lumen solves the problem by weighting the output power of a light source at each wavelength by the average human sensitivity to that wavelength (aka the luminosity function, created by a bunch of scientists asking people "is this light brighter? ...or this one?" like at the doctors office. So if you're shopping two lights, the one that outputs the higher number of lumens will be brighter (It should even have a little "lighting fact sheet" in the US, required by some government regulator).
Okay, so we know light A is brighter or dimmer than light B. Does that mean it's better? Well, that's more complicated. First, let's consider what is white light? If you've seen 'white' LED lights and thought, "wow those lights are blue", you'd be right -- they have a lot more blue than many other light sources we're familiar with, such as Sodium vapor (those very yellow street lamps), most classic tungsten light bulbs, or even the sun. What describes this is call "color temperature", and the 'best' value depends on where you're going to use the light. The temperature part comes from the fact that up until relatively recently, our only way of making artificial light was to heat something up until it glowed -- most famously, a tungsten filament. This is also how the sun works. Turns out that the output spectrum they emit is related to their temperature, so we just describe what white-ish color that looks like as "color temperature" (Lookup "black body radiation" for more on why here). You may have seen this as a menu in better cameras. Oh, and it's all in Kelvin, because reasons. The important thing is that lower is more yellow, higher is more blue. For detail oriented work, 4100K has been the historically chosen value, one that I find works well. Inside the home, you probably want something more towards 2700K, as that is a softer, warmer tone. 5000K is used in some color critical applications as it's similar to sunlight at high noon on a clear day, and higher than that it will look distinctly blue.
Think you're ready to go light shopping yet? Nope, hold on, it's not quite complex enough, let's add color rendering! This first became an issue with fluorescent, but has become much larger with LEDs. Usually, if you use the "get something so hot it glows" method of making light everything will look pretty good, if a little yellow -- this is because the glowing stuff is emitting a uniform spectrum of light. If you selectively sample two wavelengths near to each other, they'll have about the same energy. However, this wastes energy, because it also produces a lot of infrared energy (that we can't see) and a small amount of violet / UV (that we can't see / gets blocked by the glass of the bulb). So, people started making light sources that use a very efficient mechanism of making one light wavelength, and then apply a mixture of phosphors which convert some / all of that base wavelength into the visible spectrum that we see. This is how fluorescent work (but with a gas making the initial light), and how modern white LEDs work (using doped semiconductors). But, the output spectrum will have some peaks, and even though the light may have the same color temperature and luminous flux, the colors still look funky (which is why office furniture, particularly of a certain vintage has *that look*. You know the one that only looks good under cheap fluorescent lighting.). To make it possible for consumers to analyze this, the US government uses Color Rendering Index (CRI), and by extension, most of the rest of the world does last I knew. This is just a set of some color swatches, which are analyzed compared to an idealized tungsten (aka black body) source. And if that sounds tedious, don't worry it's actually done in a computer with math. The best possible CRI is 100, but a lot of lights only score around 80. It was actually introduced for fluorescent lights, and the color swatches don't look like any colors commonly known to man. With the advent of LEDs, it's pretty easy to get a good CRI score with a light that still looks pretty crappy -- the technologies have different shortcomings. To combat this, there are other color rendering units (for instance for TV lighting), and there are an additional set of swatches. Of these, the only one worth mentioning here is called "R9". R9 is just really red, like Ferraris should be, and it's particularly of interest in LED lighting because the red color is important for rendering a pleasant skin tone and red phosphors are the hardest to make, most expensive, and thus a part commonly cheaped out upon. See, the actual LED is bright blue, but to make white you have to goop a mixture of silicone and phosphors on top of it. The less of those expensive colored powders you can use, the cheaper the final product. Hence, cheap LED lights with 5000K color temperatures and 80 CRI. For a shop light, I'd just stick to things that actually make some type of metric on color rendering (or ensure that there's a regulated minimum in your locality, California requires 90 CRI and an R9 of > 50 for household interior lighting products).
Fixture wise, LEDs are completely different than previous lighting technologies because they emit most of their waste energy has heat that must be conducted away from the chip. Overheating shortens the lifespan by causing a positive feedback loop of yellowing / hazing in the silicone encapsulant. Additionally, in poorer designs, you can have a runaway thermal condition where as the LED gets hotter it's forward voltage drops, which allows more current to pass through it, causing it to get even hotter... LEDs also are inherently directional, unlike previous technologies. These two factors make all retrofit products likely to perform poorly. I've had LED bulbs burnout in less than 5mo. Any light fixture with a cover will only make the situation worse, as it's more light lost and more insulation. The LED drivers also need heat dissipation, and packaging those into a retrofit product also leads to compromises. The best designs are just sheet metal boxes with some plastic on the front: They dissipate the heat, spread the light have plenty of room for a quality off the shelf driver, and are cheap to make on existing assembly lines. B
You should be able to get more output from where the fluorescent fixtures are located for the same power, but it may not be a ton and I don't think it will solve the real issue. Fluorescents can be pretty efficient, but LEDs are over 100 lumens/watt consistently now, so if the tubes and ballast are older or lower end I'd say the LEDs are a shoe in for being an upgrade in output and quality (also: no fussing with cold start bulbs or ballast, or mercury hazards on breakage). But I think you'd actually benefit more from just having a few more fixtures of the same output evenly distribuited on the ceiling. I reckon this: that fixture is two 4' tubes, so probably around 4000 lumens if it's a 32W. If it were me, I'd see about replacing both fixtures with either 6 or 8 evenly distributed ceiling mounted four foot long 4000 lumen LED light fixtures. Batch deals on those at a glance appear to be under $200 US online, and having more light that's also coming from more places will cut down on shadows, instead of just making them starker. There are even diffuse panel light fixtures available if you want to take that line of thinking further. I'd try and loop the owner in if possible, because some decent LED fixtures will likely outlast your lease and are an upgrade that will be a bit of a hassle to move. For workbenches, I really like having a hutch, since you can put lights in the hutch that don't get in your eyes, and then also pile more crap on top. Those reels of LED strip lights are great for a budget job, or an "under cabinet light".
). That's why I don't want to make alterations that would be too permanent. 
