I know there is a free C compiler named sdcc, but it seems to be a bit too complex
I have never used SDCC, but any '51 compiler will "seem to be a bit too complex" unless you are willing to learn the intricacies of the processor.

What I realy need is a very good documentation with a step-by-step tutorial.
Nothing beats Keil in this respect. You can get a free eval version - DO read the restrictions, a search on Keil eval here will give you the links.

Other possibilities are Raisonance from France and and HiTech from Australia (htsoft). They are both good and lower priced than Keil.

A compiler like this (same price) for 8051 would be great.
I found dunfields compiler site. A nice package for a nice price. But what`s about libs for hd4470
what is hd4470?

Re Dunfield, all I know is that the posts I have seen on this product have not been encouraging.

Erik