Scotty has mentioned on several occasions that the Nissan trucks are OK. That one even has a manual transmission, better yet. However only a good mechanic giving the truck a thorough inspection before purchase can tell you if it's worth considering.

As far as fears of Nissan going under, if that happens obviously dealer service will be gone and factory parts will become harder to find. Services requiring factory equipment would become problematical. However many parts should still be available from the original suppliers, and there's always the aftermarket and junkyard parts. (I've already lived through the demise of Studebaker, AMC, and Saab to name a few and continue to drive the last two so I'm somewhat familiar with the "orphan car" situation.)

If Nissan merges with another company it's possible that factory support will continue but that's not 100% assured. For example, when Chrysler bought out AMC in 1987 one of the first things they did was to destroy all of the AMC factory parts in stock as well as the tools to manufacture them for older models.