OP:- you are right to buy it on condition alone - forget the mileage - both engines are very good, and both will do high mileages with proper maintenance - Do this yourself!
If you go the TD5 route, then get your own diagnostics - no diagnostics = no TD5 IMHO. I wouldn't have one - had me fill of electronic sh*t, and would rather not bother again.
300Tdi can be tuned to high levels - 200 HP is possible -but EGT's will kill the engine in a heartbeat unless they are watched very carefully. It can be quite expensive to get decent power out of a 300 - by the time you've got a full width intercooler, had the pump set up properly, then you'll be north of £700, and probably around 160HP - EGT's are still important - so you need a gauge, and you need to watch it too!
TD5 can be remapped reliably for sensible money, and can be reverted to standard pretty easily too.