Really depends on what you are wanting.
Camcorder style cameras have the advantage over DSLR's for continuous auto focus and this is about it. DSLR's have the range of changeable lens and all the control that you are used to with already having a DSLR. You will be shooting everything in manual though so if you aren't able to get used to changing settings quickly and even remembering to do it, it might not be for you. If you know about aperture, ISO, and shutter speed you should be fine.
5D is clearly the big boys toy and if you can afford it great. Full frame sensor is gong to work better in low light and well it's the dog bollox. As long as you learn how to use it you will be very happy with this camera.
If you don't have the budget for a full frame sensor then I don't see the point going for anything above the 600D (unless you just have money to burn) if your primary use is for video.
It does 1080p at 30fps and 720p at 60fps. It has the fold out screen where the 550D doesn't (this is actually super useful). The 650D has continuous auto focus but it's rubbish, so bad I wouldn't even consider it. The other thing is that Magic Lantern is available for the 600D which is basically some hacked firmware that allows you to do a "whole load of extra stuff". You might not be into the idea of running hacked software on a £500 camera though. I do and love it but not everyone has the nerve.
I started off with a GoPro shooting MTB stuff. Pushed it to the limit and got a 600D, IMO it will be the perfect combination. Unfortunately I broke my arm soon after getting it so not shot any MTB stuff yet but I have made a few other little movies that I'm happy with and really can't complain about the camera.
You don't need very expensive lenses at all. I have the Canon 50mm prime, about £80 (awesome!) and the 18-55mm kit lens, came with the body, (not so awesome but fine for a kit lens). Clearly a nice lens always helps though. Its something you can invest in later if you get really into it.
(By the way the Rebel T2i is the American version of the 550D and the Rebel T3i is the American version of the 600D.)