hello leon, and welcome. you say 1990, is that a mk1 or 2? and most importantly....let's have some photos!!
right down to business: leather interior...i've heard of them, but never seen one, sounds like a winner, possibly like the stradas? more photos please
welding, get used to it, rust will be your enemy, forever. and almost anything short of the glass and plastic hatch will go. have a solid poke around under all the arches, the plastic extensions, sill guards, turrets, boot floor, legs, doors, in fact anywhere where there is metal. it will come in
MOTs....ahhh, electrics has always been the thorn for me, good look at the fuse and relay panel, and all the connectors behind it, same for the lights, check the bulbs are all seated correctly, and ends not corroded. it's a simple job, but takes ages to get them all working right....eh chas?
brakes, uno T brakes are marginal at best, so a through going over here will be good, the handbrake cable snaps like a twig, and depending on how long it's been unused, discs will start to go rusty round the edges, all are quite cheap to replace.
suspension bushes, rubber, old and cacky, replacements will be cheap from somewhere like desira, or go all out and poly bush.
check the fuel pump and filters too, you have a primary leading into the pump under the car, pretty much where you sit, even if the protective cover, plastic or steel, is in place, will be caked in decades of poo, worth a clean up, and filter change. same for the secondary under the bonnet.
then it's a matter of how far you want to go
dump valves? any really, i've had bailey, le mans, etc on both mine, all the same to me. easy to fit, take you 2 minutes
as for the filter, no need to remap, what you can't anyway (if it is a mk1). just ensure you got it clear of the worst of the engine bays heat, look behind the nearside headlight, if the injector fan, and associated brackets and hosing are all still in there, get it all out. and run the filter down into there. plenty of cold air feed