I dunno I'm mighty tempted, you can pick up a 90's school bus in decent condition for 2-5K, I'm budgeting roughly 2-3K for repair and another 3-4k for modifications (not including welders and the like) and for 10K you have a long flat bed with no need for a trailer. Basically it's hard to find a motor and frame in good condition for as cheap as you can find em on school busses. I agree that a pickup is better suited for a rig welder but a similar condition frame and motor would be much more.
The DOT has released a good survey on bus length, turning radius etc
http://ntl.bts.gov/DOCS/MBTC1054-1.htm I wouldn't go for a full size (84 passenger) a 66 passenger bus would be plenty long enough
I guess I should mention that I'm not trying to make a "rig" welder, welding engineering is my field and my work vehicle is a motorcycle, but I used to do a lot of metal work all over for all sorts of things and I just thought a bus would be a fun project and platform to work on and give me much usuable area than a pick up frame.
I probably am crazy tho, my mom tells me she dropped me as a baby :-/