The lens will be more important than the camera. If you want to go with Canon, and you find the 50D too big, you will want to try out something like a 350/400/450D which are smaller.
If you are only going to use a long lens for safari, instead of buying one you could consider either buying
second hand, then selling on your return (good lenses tend to hold their value you you may not lose any money doing that (as long as it does not get damaged of course).
You could also alternatively hire a lens for that period eg a 100-400mm L from
www.lensesforhire.co.uk which would be an excellent safari lens.
If you have no brand allegiance, you could also consider offerings from Sony, Nikon etc or the Micro 4/3rds format such as the GF1 which are smaller and may suit your hands better.
If Canon, I'd go 450D and 100-400L hired.
There are also some good bridge cameras out there....
Good luck
Al