Not trying to rain on your parade, but I was in a similar situation with Aztec Gold #15 a couple years ago: purchasing a '98 Corvette with under 10,000 miles, that had sat for the bulk of its life in a
Chevy dealer's collection. And I uncovered minor issues almost immediately; you might peruse
this thread to follow my early exploits in uncovering these gremlins.
So, speaking from that experience, be at least mentally prepared for the odd, minor failure to pop up on a vehicle like that which has sat for so long: seals dry out; A/C may need charged; as others have pointed out, rubber gets old and brittle (think not only tires, but hoses, belts, etc.) -- the list of little things can go on and on. Hopefully this guy was a bit more diligent about proper upkeep during storage than Mr. McDorman was, but since he apparently didn't even think to dump some fuel stabilizer in the gas tank, well...

hnoes
The early C5's were notorious for developing an annoying seat rocking (fore/aft) tendency; apparently there are some plastic spacers that crumble with use and age, allowing the seats to slide back and forth about an inch. So don't be surprised if, when you're slowing to a stop, the driver's seat seems to slip forward a bit. It's unnerving at first, but isn't really a safety hazard unless one over-reacts to it. Threads covering the fix for this abound on the forum; just search.
The parking brake cables on early C5's have a tendency become unserviceable because (as I understand it) the grease that was used at the time became non-greasy and gummed up the linkages. The parking brake handle still raises and lowers, but doesn't readily engage the mechanism for pulling the cable running to the parking brake pads at the rear wheels. So the parking brake never really engages, and the handle just goes up without any resistance. The fix is relatively simple (clean the parking brake handle mechanism out, apply newer grease), but to do it generally involves pulling out the bulk of the center console. But this would be a very important thing for you to check, given you're buying one with a manual transmission... Until you're certain it engages properly, don't rely on the parking brake to hold the vehicle stationary!
On Aztec Gold #15, the front sway bar links had either failed or were failing; that one
really baffled me on a car with so few miles and the easy life of a garage queen...

And I recently had to have the exhaust on
both sides repaired when the flange weld on a section of pipe leading up over the transaxle came loose (see
this thread)...
In the end, none of these things would stop me from buying the car you're looking at; it sounds like a real gem! Just mentioning them so you can make an informed purchase!