Grandpa driving doesn't mean efficient.
Just for comparison on van size, our vito at work has a 1.6 diesel, and gets 45+mpg on motorways. Horrendously gutless but good mpg. I'd say any modern small diesel should be able to get nearer 50mpg minimum. I get over 50mpg with a 2.0 diesel to and from work every day, not like for like, but bigger engine and cold start twice a day.
I'd be looking for things like sticky brakes, full service, tyre pressures, etc. Just make sure it''s all in good condition.