Hey axeman,
I can only offer a personal opinion and some food for thought. I've welded over 41 years and comments from the other experienced welders here should give you some info. to consider regarding these kinds of repairs.
1.) A lot of welders shy away from frame welding because of liability and usually welding done without manufacture information usually creates more problems.
2.) Rust can be a nightmare and render any kind of welding fruitless. Metal has to be prepared very effectively to obtain maximum weld strength and safety. Also, welding creates what is called a HAZ( Heat Affected Zone), that usually softens the metal in that area, or creates a brittleness. Also can create more cracks than the original crack.
3.) The information provided is really minimal. Since the seller indicated he doesn't know much about welding, and "that's what the guy did", I would definitely pass on this vehicle IMHO. Frame cracking is a serious problem and most times with a vehicle rusted as this one is described, there are probably other undetected cracks.
I'm sure others will offer their opinions and suggestions and provide more thought.
Remember...Buyer Beware...Good Luck....Denny