Trident

Id be surprised. Its a van conversion, not many vans had any people in the back, unless a inibus which Im fairly sure these weren't.
Retrofit near impossible, been many posts like this on here

Then the person in the back has no seat belt???

We know that 3-point inertia real belts are possible in a T4-based Trident, because later ones had two fitted at the back. But... retro-fitting may be difficult.

If you want a better idea of what's involved, take a look on Ebay at some of the more recent T4-based ones - specifically ones with photos of the tailgate open.

As you can see - not a simple job, and probably not something that anyone could consider a DIY job.

And there's the problem. It's not so much finding a company that can do the engineering work - which in the overall scheme of things isn't complex. It's finding one that will sign off their work as being safe. That's where you may find difficulties.

One thing that has been suggested in another similar thread: it may be worth contacting companies that do conversion work on disability vehicles. Don't expect it to be cheap though!

Never had a habitation check, but I'd be surprised if those are things they'll check. More for "caravan" bits and pieces for safety and whether working correctly.
Mk1 eyeball first for rust in my case!

I have to say I like the way you've gone about all of this, sure the forum must have helped, but now its down to just two things, hopefully minor rust (however I'd get it on a ramp asap for a proper inspection, rust will be the ultimate job stopper.
The oil leak should be visible from the ramp too, that should be easier, certainly easier than dealing with rust.
Good Luck, here's a couple of pics of properly treated underside, it really wasn't actually rusty, but is now correctly undersealed. You could do same if you can find a garage like mine!!

Mine is hot waxoyl
Incl a good few other things circa £1000
Money well spent IMHO