One last thought - I don't know how things are in Germany in terms of
ebay, or about your mechanical skills/enthusiasm, but being on a brutally tight budget, I've always done my upgrading by buying good old bikes in bad nick, preferably badly listed, then stripping off what I needed. I just got outbid on
this one, eg, which I had planned to strip and use to build up an old frame I've just got fixed, taking the leftovers to the recycling.
It takes homework and patience, but you can get a lot for a little.