The RS owners club is the place in Britain . They can advise you on parts and costs and who but Germany is a bit diffrent . I'm sure there should be a club there for them and they can help out . Most of the modifications will be the same so no problems there . The one that will be a problem is finding a reputable garage to fit them what I would suggest is finding a specialist for cosworths , there is bound to be a few try the big cities .
Heres some others I found
Its all about the escorth but the mods are the same and a good DIY section
This one covers all cosworths escort/seirra and has an excellent link page
The Flying Haggis