Hey Jon,
I'm always intrigued with a difficult project, just to see if it can be accomplished, ...the fire box door looks to be thicker than 1/8", more like 1/4", to me the thickness of the steel would be one of the determining factors.
Frozen hinges can be freed up or cut off and replaced so that's not a big problem, cleaning off the rust can be done by sandblasting, so the mods I would do to make it work is cut off and lower the fire box, cut an ash removal door on the side and build something to hold the charcoal up off of the bottom, that way I could lower the rack and install tuning plates, if I were to go that far I would also lower the door opening so that the rack would slide out.
So I guess the most important question is how much you would have to pay for it, if the seller thinks it's a family heirloom I would pass, but if I could buy it for the price of the steel or even lower I'd take it. You said that someone else would be doing the mods, how about consider using a school welding class or as a trade school project (you provide the specs), that way it wouldn't cost as much as having it done in a welding shop, ...it might not cost any more than some meat to test the smoker when it's finished.
I hope that helps,
Gene