Do bare in mind that there is no central federation at amateur level, so if you have an idea as to where you will live, go in the Regional Verbaende section and choose the Verband you'll be part of and contact them.

I'm pretty sure you will have no problems finding games to ref, there is a chronic shortage of refs.

The football season in the amateurs league goes from early August until early December, then winter break, then it restarts early March up until June. There is not that much indoor football I'm afraid.