Since I just a bought a 98 GTS myself, let me tell you what I learned . . .
1) does the air conditioning blow cold (my did not - cost = $215 service visit for charge). Luckily it is a good time of year to test this.
2) do the doors stay open (my passenger is OK, but Driver's will not stay open). When the hinges wear out, the detent feature goes bad (it is one of the embarassingly low tech features of the Viper) and the door will not prop open (real PITA when trying to work on or get into the Viper). Only way to fix with with a new hinge which is $250 retail.
3) weather-stipping around doors. Vipers wear these out. You can cheat and color the white fibers that show back to black with a Sharpie, but otherwise they will need replacing (not sure of pricing on those, haven't order them yet).
4) interior issues: are the little cargo nets in the interior stretched out of shape? They are not expensive, but a PITA to install (upper one requires the headliner to be removed to get to the fasteners). Are shift and E-brake boots dry and carcked. Not a big deal as new not too expensive, easy to install, and you might want to have a custom color made anyway. Also check "trim bulkhead panel" behind seats, specifically the top edge that transitions into the hatch area. This is really a cheesily made piece and one corner of mine was cracked and sticking up. If you are a do-it-yourselfer you can glue it (I did), but the new part is expensive and might be hard to install. Also, are the 29 black trim screws in the interior that have tapered heads and tapered washers present and in good shape. Some of mine were missing and others rusted badly (sign of a car that was not well cared for, but I paid an appropriate price). Retail on these guys is $4.50/each. Ouch! Check the little trim pieces that go around the interior door handles. To remove them you pull toward the rear of the car and they pop right out. If someone has glued them in (they do not stay on very well) beware that there might be more "rigging" in the interior to match - especially if the car has a stereo installed. After spending a day taking my interior apart on mission to end all squeaks and rattles, I am disgusted at the number of hacks the stereo intallers left. Really uncalled for (a great argument for buying a car with the stock stereo in my book). Make sure the plastic loops that screw into the seats and hold the seat belts are there (very cheesy piece that is easy to break).
5) look for missing parts. This is kind of tough if you are not used to looking at Vipers. For example, my car is missing a piece of plastic inner fender on the driver's side the goes right behind the headlight, and I think I am missing a trim panel on the driver's side of the hatch/spare tire area (not sure about this yet - more dastardly doings by the stereo installers I fear).
6) make sure the car has clean, functioning air filters. Seems to me that lots of Viper guys are interested in playing games with air filters.
7) have the exhaust tips cooked the rear lower facia? This is a problem on many aftermarket exhausts.
If you have not already set a price, show the sellers the scrapes under the front end, and show him how badly the emergency exit handles on the "B" pillar of the door squeak, then push on the top rear corner of the interior door trim panels and show him how badly those squeak and make faces all the while about how disgusted you are. Might help getting that extra $500 out of him (which you can put toward your short gears!!).
