Along with the above I would also check the following.
1. Check the ground under the motor and the rear end for any fluid that would come from the car.
2. Look through the rims at the brakes and see if the calipers are rusted and if the backing on the pad is rusted.
3. Look at the rear end and see how badly rusted it is
4. Check the top of the motor and drag your fingers across the back of each valve cover. If its caked with oil you will need to replace them right away.
5. If there is no heat shield on the turbo look at the casing, pull the intake off and shine a light into the turbo.
6. Check the windshield, door jambs, and rear quarters for any sign of rust.
7. Look inside the trunk and tap the floor for any signs of rust.
8. Charge the battery
9. Turn the key to ON and check the lights all around the car
10. Before driving the car, change oil, tranny fluid, coolant, and check the belt for any wear.
11. Drive the car directly to a shop that knows these cars and pay them the hundred bucks to look it over. The hundred you spend now could save you thousands in the long run.
If the owner is willing to sell and the price is something you like then I would suggest towing the car to a shop. You should be able to rent a car trailer for around twenty dollars, if you dont have a car/truck with a hitch then a tow could be had with a deal from the shop you will bring it to.
Tell him you are interested and just want to make sure everything is good. Let him know the cost of getting the car towed to a shop and looked over along with changing all the fluids. Make sure he will agree to deduct this from the sale price if everything checks out.
If you find out its a turd then you have at least got something out of it. You will now know what to look for when looking at another one.
My GN sat for years and I am currently over $600 in upkeep getting it back to normal running condition (check my last thread, I am looking for injectors because of this). My car runs, albeit crappy, it runs and drives and when I get to higher RPM's it just sings. Once I get injectors and an exhaust on my car it will be perfect.
So far I have had to put tires on my car, get it washed, waxed, buffed, and wet sanded, get tires, and new brakes (also flush the brake system). Its coming along nicely but you have to know that you cant just buy one of these cars and drive it, you will need to put some money into it.
Also look at the tailpipes when its running. If you see smoke of any color then you know the motor will need a rebuild or at the very least a new turbo will be needed. If the body is in excellent condition and the price is right, a rebuild is pretty cheap these days.
