Look around at some pawnshops in your area. Sometimes they will have camera kits for very cheap.
If you're only going to get one lens, I'd have to go with the Canon you mentioned. A prime lens will be sharper and telephotos in the 80 range make good general purpose/portrait lenses.
The Pentax will have many used and new lenses out there made by several manufacturers and if you decide to buy a better body later on, your lenses will still work.
As for canon, they use a different mount than the current autofocus lenses and as such old canon manual focus lenses will not work on new AF bodies.
I'd stay away from the Rebel. You're not getting much over a manual focus camera, for a little bit more you can get a good AF body like the Elan II, if you want to start with an auto focus setup. The Minolta X series is good. Cheap too.