I just bought my 94 B4000 with 181k mileso n it, and it still runs and drives great. One thing I would definitely look at are the rear shackles and mounts. They seem to rust out on these trucks fairly often. I had a shop replace mine after I bought the parts, and it was probably about $500 total ($175 just in parts), lot less if ya do it yourself though. But if they are rusted out, use that as a bargaining chip to get the price dropped even lower.
I haven't had this truck long enough to really recommend any spots to really check out, but in general check the undercarriage for abnormal amounts of rust just like any car. Make sure the engine has no deep knocks which indicate bad pistons/cylinders. The 4x4 auto hubs are probably non-functional if they haven't already been replaced by manuals. Motor on the transfer case seems to go out too, especially if it doesn't get much use.
As for reliability, it is a Ranger. Everything on it that matters is the same as a Ranger, only difference is some sheet metal. The 4000 only comes with the 4.0L which is the same engine used in all Ford trucks/suvs. As for why the price is so much lower, people probably think they're not the same as a Ranger, thus not worth the same. I know there's virtually no aftermarket for the Mazda, though a lot of Ranger items will fit, aside from head/taillights.
I paid $500 for my 94 B4000, and it was well worth it. After fixing the shackles I was right around $1k invested. Still drop money into it, but nothing major needed fixed on it aside from that.