I spent many happy nights doing astro shots though a 1200mm f8 scope with a 40D. Mainly moon, planetary and some star clusters.
If you just want to shoot starfields then anything reasonably sharp will do. If you want to get nebulae or the milky way, then ideally you want to get the IR filter removed (or find a 20Da or 60Da- hens teeth and rare respectively) as it blocks most of the hydrogen alpha wavelengths, which emission nebulae emit. If the camera can pick up H-alpha, it'll really bring out the brightness and texture in nebulae. This does make the camera useless for general photography though unless you fit an IR filter to your lens.
To start off with you don't have to have a tripod, just a bean bag and a remote release (or use a shutter delay), particularly if you're doing wide starfields. As you know, even wide angle, with 20 second+ exposures you'll start seeing star trails, so if you get hooked, consider a tracking telescope mount, or a tripod and astrotrac.
It all depends really on what you want to capture and at what focal length though. You could even consider a scope to hook your DSLR body up to, and you then benefit from visual astronomy too, but that's a whole other conversation....