I've got a 77 BJ 40 - the B engine that's in it is pretty tired and parts are VERY hard to find. Not withstanding my cruiser tops out at about 60 kmh or so -- that may just be my cruiser, though.
I have a '79 BJ40 and an "83 BJ42. I love 'em BUT: they are not as powerful as an FJ40 with the 2F engine, top speed with the '40 and a 4-speed is about 65MPH (less if the wind is blowing from anywhere in front of you). The '42 has a 5-speed, it is faster and quieter at highway speeds if the wind or the steepness of the hill you are climbing allows you to use the overdrive.
Bottom end torque for wheeling is great, IMHO, they will serve you as well as an FJ40 offroad, and they get more than double the fuel mileage in my experience.
As for the one you saw, if it is stock Canadian spec it is a BJ42 with the 3B engine and a fourspeed, same drivetrain as my '79 now has in it. Other than that I cannot give you any advice.
Love the diesel. I have the Canadian spec (24v) BJ42 and I love it but it isn't built for speed (what cruiser is?) and parts can be harder to find, having said that, I love mine and would take it over a gasser any day.