I wouldn't quibble about tyre age and cambelt changes on a 21 year old motorhome - thats easily fixable
As has been pointed out the MOT history check won't reveal engine size but I wouldn't worry about that either.
All that could detract you from what really matters - damp.
To give you an idea of whats involved, I copy and paste a quote from Dave Newell, who reskinned a motorhome which would have been around the same age when he did it. He posted this about 10 years ago, so the prices he quotes will be far more now;
QUOTE FROM DAVE NEWELL
Being an elderly Elddis its stick built, that is to say the frame is 1" square softwood with polystyrene infill for insulation, a thin plywood skin on interior with decorative coating and a thin aluminium skin outer with thin paint finish.
Rebuilding a motorhome of this type with such extensive damp is a huge task; I know I was that soldier! I re-skinned a 1990 Autohomes Highwayman a few years back and had to reframe a quarter of the entire structure.
There are three main areas that water gets in and whichever is the one (or more) in your case will need dealing with or you'll just end up with a damp motorhome again.
The first point of water ingress is poorly fitted window seals.
Second is roof to wall joints.
Third is the worst, when water has got into the structure the wooden frame will eventually rot. Wet rot in soft woods as used for motorhome framing releases chemicals that will eat through aluminium skin in short order and will appear as tiny dots on the outer skin in line with the framework sections (often referred to as aluminium cancer).
One and two are easily dealt with but three is a bugger as it means the rotten timber will need removing and the skin replacing. Replacing the frame timbers is difficult if the damage is extensive as once you start to remove the interior or exterior skin and rotten timber the strength of the body just evaporates. I did mine one stick at a time after removing the entire outer aluminium skin (not an easy job in itself) and replaced all rotten timberwork with 1" square aluminium box section screwed together with stainless self-tappers. For the roof I fabricated a TIG welded framework of 1" alloy box section, which was placed on top and screwed to the sidewall frames.
Re-skinning was the most challenging part and took four of us three days to do the two sidewalls, rear wall and roof. Trust me when I say handling a 5 metre by 2 metre sheet of 0.8mm alloy without kinking it is not fun and spreading twenty odd tubes of sikaflex over the surface by hand is no easy job either!
Sorry if this sounds negative but I write from personal experience of a similar project. Cost wise I spent over £3K at trade prices and took nine months to do it!