Be afraid. Be very afraid.
I've had two. The first one was an automatic 1980 car, and the second was the much-vaunted 1981 5-speed. Seventeen cars out of a hundred got the 5-speed.
It will probably average out to about $500.00 a month to keep it in pitch and tune, and parts are very, very expensive. Add to that not a whole lot of shops, even with the Porsche shingle outside, will work on them anymore. If there's a mechanic the current owner recommends or uses on a regular basis, ask that person about the overall condition of the car, and make sure the maintenance records at the shop correspond with what the owner tells you.
There should be three keys, also. Two are driver keys, and the third is a red service key. If the mechanic has the service key, there's a chance maintenance is being kept up with.
Beware the mileage, also. Look at the title the current owner has, and check to see if it reads actual from the last sale date. The odos tend to die at about 80,000 miles. Since that one reads 115,000, there's a chance it's just off a wee bit.
Mileage is terrible, too. I got about 8 mpg in a well-sorted car. There wasn't a darned thing wrong with my second car when I sold it, but I got nothing back from my $11K purchase.
I'd also make sure you check a few things to see how complete the car is before you get into it. Take loose the passenger's side floorboards, under the carpet. There should be two pieces of board there, either birch or pine. Under those are the fuses and relays that govern the electrical system. There is no computer. Check those pieces of wood for water damage. If there's anything to indicate those planks have ever been wet, run away.
The steering system is a modular pod. The lever under the steering wheel is about the size of a spatula, and it should move freely up and down. The wheel telescopes and the whole insturment pod moves with the up and down movement.
If the past owners of the car have made a habit out of lowering that guy all the way to the bottom stopper, the wires in the gauge pod have probably been crushed several times, and the contacts aren't soldered -- leading to phantom indicators on the dash lighting up in ways that don't correspond to the manual -- which, in turn, can lead to unneccessary expense at the garage.
There's plenty more to look at. Let me know if I should continue here, or call me on (443) 995-0576. They're wonderful cars -- the most joyful driving experience ever -- but that car had better be well put together when you buy it or you're screwing yourself from the outset.
Having said that, the first-generation 928 is to me what the '88 911 is to Angela. None better ever.