Firstly I wouldn't worry about size, the 1st gen is not a big car my jag xj is larger and I manage to park it just fine
Also forget about the car being almost 50 , GM were ahead of the times and if you get one with power steering and disc brakes they are really quite easy to drive .
Up to you what to change but if you want something that really handles and performs there are plenty of cheap and easy upgrades that will give you what you want without costing the earth .
If you can get a car with power steering , disc brakes and posi axle but the most important thing is condition of Body! Major rust will cost more and take longer to fix than mechanical issues ....
Modern tires and shocks transform 1st gen cars and the other stuff you may want to alter is optional.
Good Luck .