The previous owner of my E had installed a 5.0L and an auto tranny. The thing is, a stock 5.0 doesn't make that much more power than the stock 3-carb motor. Luckily, he held on to the original engine and I re-installed that when I bought it.
Parts are expensive (compared to my Toyota, but cheaper than my M3 parts), but plentiful. Absolutely everything is available. XK's Unlimited, Terry's Jaguar Parts, and SNG Barrett are the best sources. They all have prices on their website.
The floors rust. The rocker panels rust. Fenders rust.
My engine is incredibly reliable. My wiring has had no problems in the 2 years I've owned it. My brakes and clutch hydraulics, are not too reliable. The original parts that have been maintained well are rock solid. The parts of my car that have been "engineered" without "[placing] too high a value on authenticity" are unreliable, difficult to find parts for, and will end up costing me more to replace than if the DPO had just not berkeleyed with it in the first place. Just sayin'.
All told, I have about as much money in my fairly original Series 1 E type as a well-equipped civic si. My purchase price was the same that my Mom paid for her new CRV, and about 5k replacing missing parts that the DPO felt were unreliable and too expensive to use.