Matty,
Here is my 2cents.
I bought the 400D new with 5 years warranty. Now i really like this camera and i have imaged in a 8" SCT once or twice. And via my refractor a few times. In the 8" SCT , I used a F6.3 reducer, EOS to T-ring adapter, and a Tring to 1.25" nose piece adapter. I did find a bit of vigneting around the edges. If you are going to do this i suggest you get a 2" nose piece adapter (that is made to avoid vigneting), and also get a 2" Visual back for the 8" SCT. The 8" SCT's only have a 1.5" opening at the back so you will not get full use out of the 2" but the CMOS sensor in the DSLR's is smaller than this so you should not have a problem there.
Here is what i have learned the hard way.
The Ha response without modfication is not very good at all. But thats if you target Nebula's with Ha. You could target other objects like galaxies etc
But I think in hindsight I would have been better off buying a 2nd hand or Refurbished 350D or 20D. Because the cost is less and you can get it modded and still spend less money than on a new camera. Also you will not lose any warranty this way that you would on a new camera.
My advise is you buy a used or refurbished camera and save some money and put it towards the modification. The 350D is 1/3 or the weight of the 20D but both have a sensor with good pixel size suitable for Astronomy.
Have a look at
http://www.adorama.com/ they have really good prices on used and refurbished camera's. They are also rated highly on
http://www.resellerratings.com/. You can also get a locally used one from Ebay.
In hindsight i would have been better of buying a used camera and attempting to either self mod it or have it modded by a pro.
Regards
Fahim