I have a 1988 petrol Talbot Express swb van. Average mpg is 23-24, best ever was 30 tailing lorries, worst ever 20 odd driving through Spanish mountains. I imagine you'd be looking around 20mpg average.
Problems, gearbox synchros go, mine has gone in second but it's not a major problem. I don't think they're worth fixing because it's expensive and not guaranteed to last long, it's easy to live with. There needs to be 1.9L of gearbox oil in to stop 5th breaking, this is above the max line on the MK1 models, I think they raised the line on the MK2's. I don't think they ever fixed the problem of 5th for another 15 odd years on the later generations.
RUST! They rust everywhere, it will be killed by rust before anything else.
Parts, mister-auto.co.uk stocks lots of brand new parts still and they're not expensive. They ship from France.
They are great vans imho. If you ever travel to the continent you'll see hundreds of them on the beaches of the med, which I think is testament to their longevity. They have no cool factor and are overlooked, but that just means you get a decent van without cool tax. VW's of the era are double the money and you hear so many people complaining about the problems they have with them.
No power steering is not an issue, only on rare occasions of being in a really tight space is it a pain, but easily worked with.
I would buy another one definitely on the condition it had minimal rust and I would treat it and keep it forever. Unfortunately, I think my current one has gone too far to spend the money on.