As stated above, if possible take someone with you who understands these cars. Also, be aware you will likely have to spend some money on your new pride and joy very soon after you buy it. I just bought a 1999 P38 DHSE, a beautiful car. However, when I got it home I realised it has some issues with the EAS and other minor problems and I had a mechanic look at mine! Post 1999 MY are a better bet as they have improved electronics (Bosch instead of Lucas) and all round traction control.
If the car has been started (engine warm) before you get there be suspicious, try to go back when it has been left standing overnight, see if it has dropped to its bump stops. I am not the most experienced p38 owner so this is just my opinion based on my own experience. Also, my experience so far with this forum has revived my faith in my fellow human beings, the help you will get here is invaluable and freely given.