That car looks to be in great shape, even down to the complete toolkit. I was looking for something to knock the price and the only thing I could find was the spare tire mismatch. The leather is worn the same way as every other sport seats. The steering wheel was swapped at some point too, should have had an airbag 4-spoke wheel. Check that the control stalks are the proper distance from the steering wheel. I'd also see if he has those parts (the old 4-spoke wheel and the airbag stalks). I don't know if any collector would want a completely OEM car, but they're good to have just in case. You might be able to knock the price down a bit due to the mods, even though they're OEM, but I doubt it.
You can get a complete build sheet if you have the last seven digits of the VIN here:
http://www.bmwarchive.org/vin/bmw-vin-decoder.htmlKeep your fingers crossed for an LSD, although it's likely as not been worn out.
The slick top cars (no sunroof) are VERY sought after. I'm glad he's going to mod some other E30 instead of a facelift car. Any E30 with plastic bumpers is climbing in value. A 325iS is difficult to find under $5K and the M42 cars have also gotten noticed. All the E30s are wonderful cars, but many other 80s and 90s cars are heading up the scale.
I do think that $5800 is a bit high, but I'm figuring on local prices. Given our salted roads and climate, prices in the Pittsburgh area are always on the low side. The fuel gauge is likely the SI board batteries, although it could also be a wonky sender or a bad gauge. If you need a new gauge, be sure you get one from a single-sender car (55L tank)...the big-tank cars (63L tank) used a different gauge.