Evo is a good package, i can't live without it.. GT class is such a natural next step after touring cars but the prize may come down.permanently... or it may not, it is a high quality expansion after all. Some lonely mods use Evo too, mostly they are related to the content ie GT cars..
Race Injection could be the answer for you, the content is at least highly supported by RD...
mrrr... Combined that with Evo and you have the best of the series. But if you can live without some of the content, it's better to buy them separately.. The combined expansion prizes when bought separately is the same as Injection prize, which is odd marketing strategy but maybe better for the customer... That way there's only one purchase, download and install..
Retro: 4,99 + GT power 4,99 + STCC2 9,99 = ~20€. If you change the GT Power to Evo, it raises the prize by ~3€ but you get tons more content.. Formula RR is free expansion BTW. It's all about your preferences but personally i would limit the xTCC content a bit, they don't have such a variance than the those expansions i suggested.
As far as the trends in the rest of the world, STCC2, Evo, Retro and default Race07 content has maybe the most servers up at the moment. Note, not a fact, just a feeling.. Someone from the staff should tell more about RD current "support list", i can bet that STCC2 is in that "list", it's new and reasonably prized...
mrrr
mrrr= someone who has bought all but that...
EDIT: This page should tell you what are trending at the moment:
http://www.racedepartment.com/forum/forums/race-07-racing-club.44/