Welcome Nils,
10 or 12 inches would be a good place to start looking. When I was looking for mine, I found quite a few between $150 and 500.
A nice 25mm or 30mm 2 inch eye piece, Should be good. Ive had good experience with Guan Sheng adjustable focus CAMERA projection lens
http://www.andrewscom.com.au/optical-telescopes