Hi
Welcome to the forum
If you have the time go to the build logs and ingest some, this will give you an idea of what can be achieved and the amount of work involved, it is possible with relatively modest equipment, Joes build is a good example of what can be achieved with a hand held router on aluminium tooling plate(http://www.mycncuk.com/threads/4513-3-Axis-CNC-router) Steel frame is the cheapest option and the welding doesn't need to be perfect to be rigid.
Use the tried and tested Aluminium Profile L shape gantry.
Use the information from the logs to make a start on your CAD drawing and then post it for the guys to have a look.
Good Luck
Regards
Mike