If it was me, I'd fix the cab that needs the bottom replaced. I have personally replaced the rear inner & outter corners, the rocker/steps, lower hinge pockets and front corners as well as small patches in the floor on a cab I'm working on now. I'm not a welder, but got pretty good by the time I was done.
If finding a better cab is out of the question because of availability (you live in the rust belt), fix what you have. If it's a money reason, save your money and then buy one, in the end you will have spent more money patching your rusty cab than it would have cost to buy a good one. I bet you could buy a good rust free cab from half way across the country for around $1000 and spend about $500 to have it shipped. How far will you get with that same $1500 in the body shop? My guess is not very far...
Good luck...
Mike B
