Yeah, you want to weld in bracing before you cut the floor out otherwise it can twist out of shape on you. Same thing when you chop a top or cut out any parts that are adding to the structure of the body. 1" square tubing should work.
Here are some photos of a guy working on a Chevy Nomad. Go down till you see the text "Steve cut holes in the floor and welded braces before cutting out the old rocker panels." and check out the pics:
http://www.linger.com/nomad/Body.htm As far as buying versus fabricating, I think that depends on your confidence in your buddy's fabricating skills, and if he has the tools to do the cutting, bending, and beading and flanging to build the part. Just a flat piece of sheet with no bracing or moulding/flanging in it is not going to be very strong.
Personally, I'd have to spring for the replacement floor pan. However, I think using cardboard for a pattern is fine, I've seen people do that before.
