Looks like a project for sure. I second the weld one side shut idea- more doors=more heat loss, and like others said that sucker is gonna EAT the fuel because it's so big. As it sits now it's really just a huge grill, so it'll be tough to "smoke" on the entire cooking surface, but you could smoke on half of it using an indirect fire setup(fire at one end and food on the other). I do it all the time on my old drum grill and it turns out great. Like Dave said adding a firebox might throw off the balance and make it tough to pull, so something to think about there. The rust shouldn't be a big deal, just have the grates blasted and clean up the outside and paint 'er up. Again, it's a huge cooker, but if you think you need the capacity... For the right price,(see:CHEAP) I don't think you can get too beat up by getting it. Keep us posted.
UDS'-55&16, Akorn, Bar-B-Q Kettle, Drum Grill, miniWSM, cobweb-covered gasser. I'm JD.