I've had a couple of em, lovely cars to drive but can drain you of your hard earned cash quite easily. Fat burners are in my opinion very underpowered, a lot of car weight for only 134BHP.
Air suspension even though a pain when it goes wrong is a doddle to fix, and is just a case of plumbing. Things to look at there are the the suspension bags. If they are old they start to perish around the folds but are easy to replace. Height ride sensors can be expensive to replace if they go wrong.
BeCAM electrical faults are very common on early P38's and can render one useless if they start to go wrong & are very expensive to replace. Always check that all electrics work on test drive & that the central locking and immobiliser works without fault & that you have the EAK in the wallet.
Elderly P38's who's Air-con hasn't been looked after can also cause problems HEVAC unit with throw up loads of faults.
Mine was the 4.6v8 LPG converted and would only see at the most 23 mpg with a feather light right foot. That one went west when the head started to go, another common problem - overheating.
Those are just a few of the faults I encountered and I was able to rectify most of them myself. That said I absolutely loved them and would look at getting another one tomorrow if I didn't do starship mileage. Great on road presence, very comfortable, safe (had a rear end shunt in mine wrote the BMW off that hit me & the P38 had only a bent tow-bar) and versatile. Love em to bits