I'd say go Ikelite. Excellent housings, great port selection, fair pricing and fabulous customer service and support.
Ryan at
www.reefphoto.com ryan@reefphoto.com can help talk you through what you will need to get started.
The 28-75 if there is a port for it, might make a good recon lens. It won't do particularly wide and you won't be able to get super close ups, but it's going to work OK for middle of the road type stuff, I think.
Before I jump in and say "check out buying X" tell me what you like to take photos of as that is going to narrow the choices down considerably.
Also, you are going to HAVE to buy a strobe with this set up so make sure you budget for that (don't forget the synch cords).
The other option is to NOT house your 20D and head for one of the great digital cameras like the Oly 5050, the Canon A series, Fuji's 900 or 810, the newer Oly 350 etc. Faster learning curve on these babies and a small fraction of the cost. Don't think they can cut it against the big boys?
http://www.underwaterphotography.com/photo-contest/WorldChampions/2006.asp
Also check out the results in the SB gallery from calypsonick, PhotoTJ, SeaYoda, Dee and many others... The shots in my gallery at
www.OceansByAnderson.com are all pre-dslr, too.
And the whole thing clips off easily, too
DSLR is great...but it isn't the be all, end all IMHO.