So actually I collected some information what equipment is useful for DSLR-video- and photographing. Additional lenses are essential because light- and room-situations often change. I think a fixed and short focal length lens is easy to handle and usually has a low f-number, so it will work best in dark and small rooms.
Another important purchase will be a battery grip, because shooting video will swallow a lot of ernergy. I decided to use Sanyo Eneloop extra high capacity AA cells, so I won’t have to spend 40 Euro on a second Canon battery. En plus it makes the cam heavier, so the pictures may be a bit smoother.
So with 28mm lens and battery grip my 550D purchase will be about 1050 Euro. This is going to be my financial ruin. Don’t want to think about the other stuff I painted on my later on purchase plan, haha.