Chris,
You only need to change the belt every 5 years, so it would probably be best to take advantage of Loki's kind offer of a loan of the crank holding tool.
I think Loki meant to use the oil filter tool to compress the belt tensioner - I use a pair of plumber's pipe grips that work just fine, and a small screwdriver that is just the right size to lock back the spring. The tensioner doesn't move back very far, just enough to release the tension on the belt and allow it to be slid off.
Changing the cam belt is pretty simple business, just take your time and mark the pulleys as Loki says - the only really critical one is the ancillary shaft as the crank and camshaft have easily visible alignment marks. Get a cam belt with alignment marks for the cam and auxiliary shaft printed on it - that will make everything easier when you put it back together.
You don't really need to remove the radiator, I don't bother (but my car is an auto so I don't want to risk getting an ATF leak) - you only need to see the crank timing mark - which isn't all that difficult (and help yourself by marking it with some new tippex before you take the old belt off).
Changing the coolant is not a bad idea anyway, I ran the RB with water in the cooling system (for about 3 weeks) after I changed the head to make sure everything is sealed up properly - which is fine as long as the pressure cap is fitted (otherwise the water might well boil). If you are going to change the coolant then you might as well back-flush it according to the LS method, then re-fill with EG/water mix (I buy ready mixed EG/water - because it was cheaper and I'm a tightwad).
You can buy a brake fluid water contamination tester for about a fiver from eBay etc, one like this is fine:
https://www.ebay.co.uk/itm/Car-Brake...gAAOSw7z9fIEHI
... they all work much the same so there is no point spending any more. It will tell you whether you need to change the brake fluid or not (I check all my cars and bikes every year - it is a really good investment). DOT 4 doesn't wear out as such with the tiny amount of movement in the brake lines, but it does absorb water and get contaminated. If yours has more than 2% water content it is worth changing: a litre of DOT4 is less than a tenner (it is all much the same, don't bother to get anything more expensive or you will be buying an expensive label) and you can do it yourself pretty easily. Get your son to help and you won't need any special tools - just a piece of aquarium pipe and an old jam jar (if you are going to do it alone get a vacuum pump for about a tenner - makes the job easy and tidy).
It is probably worth going through the service checks in the BofH yourself. There is no need to change things that are obviously okay (like belts, hoses etc) or you have tested out.
I think you said you were going to get new tyres, I think that is a good idea. I replaced the 4 odd make ones on the RB with the correct size own-brand ones from FI Autocentre - a good investment at £150/set I thought. Tyres are a pretty important safety thing, if they look a bit old I'd say change them. The same goes for brake pads, they are really cheap and easy to change, it just isn't worth taking a risk with your son's motor car for the sake of £20.
It is worth checking the engine and gearbox mounts - if they are the originals they probably won't be doing much mounting. They are really cheap (generic ones that fit dozens of other cars and vans are about £6 each) and not all that difficult to change (well, the passenger one is easy, the driver's one is really fiddly). That will probably make a lot of difference for next to no cost.
It is an older car, so you will almost certainly find a plethora of other things that aren't right - that is almost inevitable and part of the fun (in my humble opinion) of running a 30-40 year old motor car.
Good fortune,
Alan