Check to see if the SGT has an autoguiding port. If so you may want to consider picking up a ToUcam as well for autoguiding with Craig Starks
PhD Guiding. With the ToUcam you can also use
Craterlet for planetary and Luna imaging.
DSLR Focus won't work with the 400D, Get
FocusAssist for free for focus and Craig Starks
DSLR Shutter for exposure control
You will need a piggyback bracket to carry your camera on the C8 but you may want to consider a rail with sliding adapter to better position your camera for balance. See image for example of rail and bracket.
If you want to control your camera with bulb settings then you will need the serial to bulb pin adapter (plus a USB - serial adapter unless your laptop has a serial port). If you so a search on the site there are quite a few links on where to get them from or how to make them.