Hi LM
Welcome to the forum.
The car looks nice and tidy for an early example of a mk2 gti.
Mods are a little bit more difficult on the early 8v's as they had the k-jetronic injection system. That means they can't be chipped like the later digifant 8v's.
The easiest things to do would be to fit a k&n (or similar) replacement panel airfilter and consider a performance exhaust (if it wont get you into trouble again), if you then fit a longer duration camshaft that would make the most of the other mods and give you smoe sort of power gain.
Pricewise I guess that car would be worth about £600-1000 in this country, I think that's about 850-1400 euros.
Hope this helps
Toby