If you google for data on any camper with a Fiat Ducato chassis of a similar vintage (probably 80% plus of all campers then and now) you will get an idea. You are either looking at a 1.9 or 2.5 diesel or a 2.5 turbo diesel. The 1.9 is low on power I believe.
I run a 1989 Hobby with a 2.0 petrol engine, very reliable engine on a par with 2.5 diesel. Always get 20 MPG, 22 on a motorway with a light foot. I think the diesels mentioned do between 25-30 MPG.
One thing about the Express, as this is the British right hand drive camper the gear linkage had to be converted from the original LHD design and goes through a complex arrangement which could be troublesome so make sure all the gears work for you.
The main thing with campers this age is trying to keep them dry, check all the cupboards up in the corners for leaks and negotiate accordingly. Good luck and happy camping.