You're thinking along the right lines. Keep a grand back for inevitable repairs. If you hunt around you can find a decent one for £2k and use that grand to get her right. Engine runs well on LPG and that makes running costs comparable to the oil burner but way more refined.
Need Nanocom or similar for diagnostics. As far as I'm aware Mongoose does nothing on the P38.
GEMS was earlier petrol engine which I think used Lucas engine management? Thor used Bosch. The intake on the top is different and there are other minor differences. General opinion seems to be that the Thor has a bit more oomph. Neither are designed to be racers though.
Air suspension niggles are common but can be fixed with the help on here fairly cheaply.
Badly maintained cooling systems will totally stuff the engine. Head gasket failures are 120k miles are common but easy to do yourself.
Headlings will drop but the fix is easy and pretty cheap.
RF receiver should be gen 3 or you'll have battery drain issues, usually accompanied by sync issues and door lock sets burning out.
The petrols tend to be prone to oil leaks. This is part of the anti-corrosion protection system. If you want to get rid of all of them it is usually an engine rebuild from scratch job!
HEVAC blend motors can play up and are a PITA to replace.
There are heater o-rings by the transmission tunnel that go through the bulkhead that are a bit of a sod. There is a mod and a bodgit, of course.
Auto gearbox needs regular fluid and filter changes and all fluids (diffs, transfer box) need regular changes.
In tank fuel pumps fail every 10 years or so and are a bit of a sod to replace.
On later models the twin exhausts force the brake pipe run behind the fuel tank where it is bathed in dirt and trapped moisture and rot to buggery.
Later 4W traction control models seem to have ABS issues more but it more than worth it off-road or in snow.
Petrols seem to suffer from sensor issues with age: crank sensor, MAF sensor and O2 sensors and more importantly only original seem to work properly so can get a bit pricey.
Someone way more knowledgeable will be along soon no doubt.
@Saint.V8 may even have done a buyer's guide somewhere on here. He's certainly answered the question enough.