Hi Kosh,
You can mount a DSLR directly to an GEM mount by using a dovetail type bar and a suitable bolt to hold it in place.
You would, as you know, first have to polar align the mount using a scope.
Also you can buy rings that would fit your scope and piggyback your camera/lens on top instead which would later enable you to guide your camera/lens using the scope and a cheap guide camera.
If you choose one of the new Canon models, (450D/1000D) you'll be able to use the live view mode for focusing which will be a big help.
Also these new models have 14 bit image processor over the other older models.
I'd probably recommend the 450D over the 1000D, since it has slightly better specs.
The 300D/350D are good but don't have the live focus capability but they are relatively cheap now as you can only buy them second hand and you can use software like
DSLRFocus to control them (up to XP SP2) or you can use
ImagesPlus as well which supports Vista.
If these were my choices I'd go for the 450D and some good lenses.