Check it over before offering him anything. I'd look for thin metal where the rust holes are; check the "propane fired" portion, it doesn't show in the pics; make sure all the doors and hinges work properly and seal well, if the material is too thin, warpage will be evident.
Not much of a trailer, only "okay" as a smoker--it depends on whether or not there is a baffle or something to direct the smoke to the cooking surfaces. If not, it's more like a glorified UDS.
I'm pretty stingy with my money--I wouldn't spend much over $300. If he wants more than that, a better deal will always come along. The problem is you have to be patient. Sometimes, I've waited a couple of years for that majic moment. It's hard to wait that long. Usually, I've had the chance to save up more money than I had and end up with nicer stuff (at a better value) in the long run.
Keep us posted.