A motorhome should at minimum contain beds, cooker, kitchen sink, toilet and a shower. They should really contain everything you need to go to a remote site and live indepentantly, like a fresh water tank, a waste water tank, mains pickup lead. Motorhomes come in different sizes and have names / classes based on there design.
Check your driving licence, if you passed your driving test after 1997, you may not be able to drive the larger motorhomes.
Check the unladen weight of larger motorhomes, if its greater than 7.5 tonnes, you may need a HGV licence.
A C Class or Cab Class is a rolling chassis conversion, basically the manufacturer takes a common van platform and adds the body to a rolling chassis
There are a lot of insurers, you can limit mileage and agreed value to keep the premiums down.
Beware that self conversions may not be insurable, check before you buy. Some people end up just insuring the van alone.
Not an easy subject. If you buy from a dealers' forecourt, they will give you a condition report, otherwise you can get the motoring organisations to inspect for you, some dealers will inspect a third party vehicle for a fee.
If all of that is too expensive, get a mechanic to check the mechanical side as you would a car, then get the seller to demonstrate everything.
Look in the interior corners and at the back of cupboards for discolouration or damp.
Look in the bottom of cupboards at the carpet/floor.
Look at the trim at the corners of the bumpers for scuff damage.
Run showers for 5 minutes and check they continue to run hot.
Check that the fridge works on 12v and Gas and Mains.
Check for waste water leaks.
Check for any smells of Gas, to get this fixed go to a Gas certified dealer.
Light every burner and check it burns blue.
If there is a heater, check it works, after you buy get it checked by a Gas certified dealer.
Check the interior lights work and what battery they are connected to.
If there is a switch / fuse panel, get an explanation and demonstration of everything.
Check that there is a spare wheel and the tools/jack to change it.
Ask where the leisure battery is and how it is charged.