Hola and welcome

,
The PD100 is a good choice of engine - economical with a very usable amount of torque, but also has a lot of potential if you're prpepared to spend some £££.
On the 9n platform itself, beware of dodgy front ARBs on cars prior to 2003. Coil springs and bushes are also notorious for snapping and wearing out after not too long. I'd suggest uprating the springs just as a preventative maintainence whilst adding powerflex/Seat cupra bushes all round to prevent problems with bushes whilst further improving the feel of the car.
As far as the engines go the 1.9 PD TDis are pretty much bomb proof. The only thing I'd look out for is that the cambelt has been replaced in the past 4 years or 60k miles, whichever's earliest - obviously the interval when it needs to be replaced for future reference.
Not really sure whether you want to do any mods either power wise or cosmetically. I'm sure the rest of the 9n crew will be able to help - I best get to the library and prepare for my exams
Good luck!
Ash
