Hi mate,
Which engine is yours? 1.9 XUDT or 2.0 DW10 Hdi? I'd Imagin its a HDi if it's a 2000 year.
Remap will yield anywhere from 115-130 hp depending on what company you go for adding an intercooler and a performance exhaust and remapping after that should get you to 150-165 hp Also disconnecting the EGR valve is a good idea to keep the inlet clean. De-cat will also help but due to changes in the MOT for Jan 2012 this will become illegal on our cars so other means will have to be sort like smashing the gut out of a standard cat. Still only smoke tested so they won't know it's a gutted cat.
after that then serious mods are needed and also serious money,
Things like:
Piper cam (they do 2 profiles for DW10 Hdi engine)
1.6 HDI fuel pressure regualtor
BMW 330d injection/fuel pump
Merc AMG injector tips
Some kind of bigger turbo
Water/meths injection
Propane LPG injection
plenty of options but not cheap really
also to note, stick with the standard airbox and OEM spec paper air filters as they are the best thing for it, all else is a waste of time and money (especially drop in panel filters)