I had my rig inspected last year before I bought it. The inspector did a fairly good job but did miss some important issues. For example, he missed that the Zone 1 heat exchangers were not installed correctly and did not work well, House battery water was very low, bay heater did not work, etc. So for the cost, not sure I got a fair deal.
The one thing I did have done which I think is critical is to have samples of all fluids taken and sent to a lab for an independent analysis. This included engine oil, engine coolant, transmission fluid, generator oil & generator coolant. This will tell you if there is any abnormal metals or compounds present and can forewarn you of potential problems.
If there is a next time, I might skip the inspection and do it myself but still get the fluids sampled.