There are not many around, so choice is limited. As always, unless you intend to undertake a major restoration, buy the best you can afford. Take a good look at the sills, windscreen pillars, scuttle, front and rear suspension top mounts, floorpans, quarter panels, wheelarch returns, leading edge of the bonnet and the tailgate. Fortunately, the Imp is well served in terms of panels, although some van-specific ones aren't available.
The engine has an unfair reputation for reliability. Well-maintained examples, with a good radiator and water pump and the correct coolant and oil/ filter will do lots of miles. It's a mechanically quiet engine, so any valve; cam train or chain clatter will need investigation. Do check (when the engine is cold) for oil/water emulsification and for smoke on startup,…