First look hard at the frame. A lot of cruisers up there have really severe rust problems and the frame is not immune. Some in ebay have been pictured with holes easily large enough to put a fist through right in the frame rails! Look at and behind the spring hangers first.
As to the engines - there are a lot of people who can speak to that better than I can as I've only just aquired one of them. Wayne, known by "crushers" in here can be particularly helpful to you if you happen to be lucky enough to catch him in a very rare civil mood
. The BJ40 and BJ42 will have either a 'B', a '2B', or a '3B' depending on year model. They're a family of four cylinder engines mostly made by Hino, a subsidiary of Toyota. Generally they can last a very long time but watch out for signs of having been overheated as that, along with running low on oil, is likely the biggest killer for them. They're solid lifter mechanical injected engines so expect a nice bit of racket that isn't necessarily harmful.
There's much more, but I think a lot of people would be happy to buy that truck at $800. for the driveline alone - engine, trans, and that.
The vehicle you mentioned (1978 BJ40) is 25 or more years old it's an easy import as it would be exempt from EPA and DOT requirements. Newer than that can be problematic.