I had a steel frame Bianchi in the mid-90's with no complaints (other than it hurt when a 40mph car used my right thigh to bend the top tube in half). It had a 54cm frame which was really 1-2cm too small (~5'10", 31"-ish actual inseam).
https://www.bicyclebluebook.com/ exists for bike price estimates, but I have no idea if it is considered accurate or not.
Based on a quick look at the local CL, I'd say that '98 Bianchi includes some markup for all the matching Celeste Green bottle, cages, and clipless pedals, but overall it does look to be in very good condition and it is not massive price gouging but also not super screaming cheap.
IMO, since you don't sound like an "it's classic Italian or it's crap" aficionado, that seems like a lot of driving for a good but maybe not exceptional 24 y.o. road bike. I don't know enough to make a good comparison, maybe someone asking $18k for a 1990 Alfa Romeo with 70k miles, factory hard top, and service records from the last of 2x owners? Classic Motorsports rather than GRM Challenge? If there is something about the bike you really like, it does not matter what some rando on the internet thinks; all values are relative.
Basic (obvious) stuff to check on a used bike: frame damage (esp. cracks--not likely-- or a badly tweaked derailleur hanger, although steel can be bent back and/or repaired), are all the bearings smooth (wheels, bottom bracket, headset; slop is not good but may fixed with adjustment, notchiness means something needs to be replaced), slop or wear on drivetrain components. Poor shifting may be simple adjustment, or it may be a tweaked der hanger and/or worn components. Wheels should be true and have even spoke tension, with no flat spots or bulges in the rim. There are some old-school techniques like holding the front brake and trying to roll the bike back and forth to check for slop in the headset bearings; hold the rear wheel off the ground and put your ear on the saddle while spinning the rear wheel to listen for bearing noises.
If you have a local bike shop (or any that are closer than 4hrs away, or are even along the way driving to see the Bianchi), test ride anything and everything they have close to the same price range, new or used, just for comparison (however, the standard wisdom is "never test ride anything you cannot afford"). There are going to be new lower-end road bikes for $500-600, or you may find you like something better than a classic drop-bar road bike.
Bikes Direct mostly has decent bikes for less $, and for $500 you can start to get into aluminum frames and mechanical disk brakes (but look very carefully at what you do and do not get, like no "STI" combo shifter+brake levers, and of course no chance to test ride first).
Depending on where all you want to ride, a hardtail (no rear suspension) mountain bike with minimal-tread XC tires or skinny slicks might be worth considering too (or it may be an overly heavy compromise you don't want). Wider 700C roadbike tires fit on "29er" MTB wheels. OTOH, just start with a road/gravel bike if you want drop bars. And of course the frame geometry is different.
Component groups: you pay for lightness, smoother shifting (specifically under load), slightly better durability, and elitism (seriously). More # of speeds means less gap between gears and/or a slightly larger gear range; OTOH a single front chainring ("1x" or "One-by" drivetrain) with 10, 11, or 12-speed rear has the advantage of completely eliminating the front derailleur. For MTB stuff at least, newer lower-end components can function nearly as well as older higher-end components, are significantly cheaper, and only marginally heavier/ less durable.
