Are the mods you immediatley fitted very expensive though?
cambelt is a routine thing about a 500 quid service from a specialist should be done every 3yr 36k.. (alfa state 72k DO NOT LISTEN TO THEM)
Cambelts tend to snap between 40k (for the unfotunate ones) and 80k but they nearly all snap before the qouted 72k which is a disgrace to be honest.
Diff is actually only 530 fitted, more and more gta owners are getting them fitted now. Its basically a new alfa part been introduced to cope with the higher torques the new diesel engines are producing (thats what puts the strain on the diff). But you got to remember the 530 is not just getting you peice of mind for reliability, your getting a LSD that can actually comfortably put down 250 bhp through the front tyres!
So you can get a decent 04 gta for as little as 7k +1k for mods you have a reasonably reliable rapid italian hatch... (am i temptng fate)