while a bit off the subject at hand, i have had both a C4 and a C3 (had a 93 convertible and now have a 78 pace car). i found the C4 to be a bit more homoginized and "car-like" less easy to work on and lots more electronics to go south on you (i never had much of a problem with it, but it certainly lends itself to some electrical nightmares). the C3 is much more of a classic style (in my opinion) and more of corvette in my opinion as to the ride, handling, etc. all the preceeding however is opinion and nothing more.
now as to your original question, does the current owner have any documentation with the car?? i recently came across a 79 C3 for sale by the original owner here in south florida. he had every record for everything ever done to the car from day one. on top of that, the man didn't believe anyone but a chevy dealer should touch his car, so all the work was done at a large local dealership in the area. his reason for selling it was that his wife no longer was comfortable riding in the car (they were getting up in age a bit), and he hated to see it just sit and be used sporatically. the car was a sand beige color in and out with t-tops. the paint was original and probably could have been made "driver quality" very respectable with a bit of work. the seats had been redone, but the rest of the interior was original and in very decent condition. the rugs showed their age, but were servicable. EVERYTHING WORKED (gauges, windows, air, etc). the engine compartment was all original, and seemed to be all there (it was an L48, automatic with air if i remember correctly), not filthy, but certainly not show quality (not even local show quality). the radio had been changed out somewhere along the way to something more modern, but that appeared to be the only non original item. original aluminum wheels, etc. ASKING PRICE $11,000.00 which he said was a bit negotiable (he hinted he would take 10,500, but no less). price is a bit geographic, and as an original, 1 owner car with complete documentation from day one, including the original order sheet when the car was ordered and window sticker when delivered, down here in south florida, that price is a bargain.
hope that helps you out some as a means of comparison to what you are looking at. again, as an opinion only (i have owned C2, C3 and C4 cars), i would certainly consider the C3 ahead of any of the earlier C4's for sure.
steve