Rod your post processing is only a start with webcaming. Though I'm not experienced enough yet to fully appreciate it I'm sure some of our more experience brethren and sisters can fill you in on how much time can be taken in processing any digital image no matter DSLR or Webcam.
Yes you will need a different "t" ring with the Canon to the Nikon. You will also need a "t" adapter as the "t" ring won't fit straight onto the back of your SCT
The canon only looks "plasticy", its feels very robust.
As far as upfront expenses, welcome to astrophotography. You have a huge array of filters, off axis guiders (or as celestron call them radial guiders) for guided photography (needed for longer exposures) unless you go for a guide scope.....
Just tell me when to stop. Seriously though you can spend as much as you want to. Get the "t" ring and "t" adapter and your right to go with taking multiple image to about 3 minutes and stacking them (if your drive axis are trained etc).
I haven't done a great deal myself, I'm crawling with the webcam first. I have done a lot of looking around reading forums, personal astrophotography websites etc though, so I am getting an inkling of how expensive it can get if you want to really go crazy. But that doesn't mean it has to cost the bank.
Consider a webcam, your DSLR "T" ring and "T" adapter, a good barlow and go for it. Like me you'll pick it up as you go.
I'll leave the programming stuff to other more experience minds, but Photoshop seems popular along with Registax.
Good luck