I agree with
@Octoparrot carry out maintanence work first like PCV, diverter valve, cam follower, servicing etc. No idea what your fault is but if you have a look on here for the fault code you might get an idea once you get the fault code via VCDS:
http://wiki.ross-tech.com/wiki/index.php/Category:Fault_CodesThen would be a good base to mod - If you go for cosmetics externally my choice would be to colour code the black plastics and maybe change the wheels if you don't like the look. Internally you could do things like head unit upgrade to RNS510 (With canbus change if required) and highline dash upgrade and MFSW buttons.
Performance mods a good start would be stage 1 (R-Tech are one of the most recommended guys for remaps) and a exhaust system (BCS and Miltek seem to be the most preferred options I found) the rest is up to you how far you want to go
Welcome to the forum and enjoy your slippery slope of modding your GTI!