Addendum: Ive not seen underneath but im quietly confident. I've saved it from leaving the family for now, but if it turns out to be more work than reasonable, i can look to move it on.
Looks promising! Yes, your first port of call should be to get on your knees with a decent flashlight and have a proper look at what's underneath. Externally, these things can look great – yet be absolutely wrecked underneath. It looks to have been relatively well stored and looked after at some point, so I doubt that's the case, but it's best to go in with your eyes open. Strip the boot, etc, nose around under the carpets... you'll soon find out what's what.
Otherwise, these things are mechanically pretty straightforward and the fact it's LHD (although odd) won't make much of a difference to the Haynes and workshop manual.
You mentioned that it runs; the M30 in these is a dead tough motor and will take all kinds of abuse – and has hydraulic tappets, so you've no valvetrain tweaks to worry about, but it's worth dropping the cam cover off and inspecting the oil spray bar bolts as they can back off, resulting in reduced lubrication and excess cam wear. No timing belt to worry about, either...
I'd do the following, as your starter for ten:
- Inspect underbody, hoses, lines and structure for corrosion, damage and leaks
- Sling a fresh battery in it/charge the battery and see which electronics work and don't (test all the switches, lights, etc)
- Drain the oil and put a new oil filter and fresh inexpensive oil in it
- Check/top up the coolant, trans fluid, brake and PAS fluid
- Put a little fresh fuel in it and some fuel system cleaner
- Check the air filter housing for debris
- Run the engine and, if nothing escapes from it, run it up to temperature and see what it does
- See if the brakes are there by standing on the pedal
- If all appears well on that front, see if it'll move under its own power (just to make sure the trans isn't junk) and steer/stop*
*you may have already done some of this, and I'm no doubt preaching to the converted, but just for reference!
My inclination then would be to, for safety's sake, rebuild the brakes – unless you can ascertain that they're serviceable and sound – and then change the ignition consumables, ancillary belt and every other fluid and filter (trans fluid and filter, brake fluid, PAS, rear diff, coolant, fuel filter, air filter, etc), pump up the tyres and get it transported to a garage for an MOT test.
If the brakes appear okay you could just otherwise service it and send it to the test centre; you could spend a fortune on fresh tyres, brakes, suspension parts and the rest – but if it subsequently goes in and fails spectacularly on something you hadn't spotted, it could get boring quite quickly. As a result, I'd try and stick to the minimum for the time being if everything appears reasonably safe and serviceable, particularly given that some bits can be quite costly.
Also, it looks like it's on E34 Basketweave wheels, which avoids the problem of costly TRX tyres (off the top of my head). Worth checking that, though.
The cost of the exhaust system might sting a bit, though. Have a proper gander at what the issue is and then go from there. I think an original is in the £900 range but at least that'd fit perfectly (but RHD and LHD systems might be different, so you'll need to look that up). Plenty of options, however, in any case.