Hi
I am not sure if P5 wheels have the same stud pattern as a P6, but if you put a bigger wheel on you will need to put on a lower profile tyre to reduce the wheel's overall diameter - otherwise the speedo will read incorrectly.
2000 wheels will not fit a 3500, as the wheels were more dished on the 3500 to accommodate the larger brake calipers. Best bet is to obtain a set of new chromed extended wheel nuts (Wadhams has these I think for about £40.00) and a set of Rostyle hub caps fitted to the 3500S.
Good luck
Gary