@Jon: nice writeup
@48x24x48: I think this hint was what I needed. Once I got the point I started to think and figured that maybe this kind of controller is available in a slightly more professional setup. Then I looked at Ebay and figured that I can get a professional PID process controller for a similar price (or much less if I either drop autotune or automatic ramping). However this would need some additional parts (PT100 + SSR). Still much better than doing it on my own.
Now I am left with some much better choices than the cheap oven 
Thanks a lot.
Udo