Bus conversions make wonderful motor homes, but they are really unlike most other commercial motor homes out there. True, house systems will be built with components and appliances common to all motor homes, but the mechanical systems of the vehicle itself are going to be quite different. They were built to go millions of miles carrying passengers, which is why we chose a bus conversion over a traditional motor home. We wanted the road worthiness that comes with a bus chassis, but of course that brings with it a whole new world of things that a traditional motor home doesn't.
There a few really active bus conversion forums out there with a wealth of technical information and experience maintaining a bus. You can also find information on Facebook, but the really good stuff is on the main three forums which are filled with owners of bus conversions, both vintage and newer. It would probably be a good idea to get on one of these to learn the specifics of what you need to look at when evaluating a bus conversion for purchase. Bus Conversion Magazine, Bus Nut Online, Bus Grease Monkey. There are some great threads which talk specifically about evaluating a used bus conversion, and while many are about older more vintage models, the basics of the lists would apply to a newer one as well.
You've got to look much deeper than just the miles on the odometer. There are some high-mileage coach in great condition, and there are some low mileage coaches I'd run away from rather than consider buying. This is where having someone with you that is experienced in bus conversions when you evaluate will be really important, or be able to get it to a shop that does bus maintenance for them to do a thorough pre-purchase inspection.
To me, the number of miles on the vehicle isn't as important as how the vehicle was driven and cared for during that time. Mileage is one of those things that can go either way. Too many miles, especially if the appropriate ongoing maintenance hasn't been done, means that you'll be in for some repairs and have a bunch of deferred maintenance to catch up on. Some bus conversions are done on retired passenger vehicles, and they will have gone many miles before being retired. The condition at that point will depend largely on how well the company maintained the vehicle and whether they were southern or northern buses. It's not uncommon for charter companies to stop doing routine maintenance on buses they know they are going to sell off. Buses running in the snow belt states will deteriorate quickly due to the constant exposure to salt, so use caution if you're looking at a conversion that was a bus in the north.
Too few miles can also be a problem. These vehicles were built to be driven, not parked. We bought our coach with about 41,000 original miles, which created its own problems. Our coach led a very pampered life, but the extreme low mileage meant that it wasn't driven often enough to keep all the mechanical systems in good shape. We've spent a considerable amount of money replacing seals that have dried out, and we've had our fair share of maintenance/repairs necessary not from miles driven but just age. It's possible to find some bus conversions like ours which were done on new shells and which have low miles on them, but which were owned by people that either had no clue how to care for them or that just didn't care. So, you have to do a full inspection regardless of the miles.
Yeah - if the conversion is 15-20 years old expect some of the house components to need updating or replacing. In the grand scheme of things, it's a small thing though. If you find a conversion you like and that meats your needs, nearly everything can be easily upgraded to modern equipment. Depending on how the conversion was done, it could be quite easy to. Many conversions provide great access to the various systems and have wiring chases which are accessible. Some were built without a thought regarding future maintenance or upgrades. Look carefully at how things are put together so you can determine if you will be able to access things you need.
Chassis electronics are much more difficult to upgrade though, so it would be important to confirm that the engine & transmission are still supported. Some newer systems also use proprietary computer systems and make doing the work yourself difficult. I'd assume that anything within the last 30 years is still supported, since many charter companies are still running coaches that old in commercial service. But it would be wise to confirm this with a shop that works on buses so you don't get stuck with an odd duck or an orphaned computer system in your drive train.