Hi Dave
Might I add something to what has already been said. With your scope and camera, I would not even think of using 1 1/4 camera adapters etc. With the 20D you will get a huge amount of vignetting if you go along that route, make full use of the chip size and go 2".
I use an SW ED80 as my guidescope and have found that you need a tube extension to bring my autoguider to focus, I have attached my 350D as well and it also required a 50mm tube extender to bring it to focus, Bintel has these at a reasonable price.
I, personally, would not get a Barlow, in fact, I would more likely opt for a 0.8 reducer to give an even wider FOV and faster exposures.
The native f/7.5 of the ED80 has shown itself to be an awesome imaging scope with a huge number of excellent photo's being published so I would just use that to start on your astrophoto adventures, worry about a Barlow or Focal Reducer, later.
Cheers
JohnG
