Sorry for the late reply. When we were going to go and get our DP from Texas, I stopped by a local (newer) Diesel shop and asked if they could give it a once over and let me know if I needed anything. They said sure...for a nominal fee.
When we got back with it, we dropped it off and said take your time. Three days later they called, apologized for the delay and gave me a short list of some issues (air leveling valves and a few others).
They said it was a good purchase and after fixing the minor issues to just drive it as it needed nothing major.
This cost a whopping $0.00!
I brought them a large case of cold Corona beer for their kindness and trouble.
You may find a place like this...or not. If they would itemize what the $400 buys you, it might be worth it. For that price, I would hope they would at least change the engine and generator oil and filter and inspect, tighten and lube everything else.
Bottom line, I would check everything out for myself (correction, yourself) and if your gut feels good, go for it. Most maintenance can be handled with basic tools and knowledge (oil, filters (diesel filters require a little more care to prevent air entering into the system) greasing zerk fittings, checking for leaks, tight belts, correct coolant SCA levels (diesel only). The rest would need a shop (heavy mechanical, wheel balance and alignment etc).