Building an AR isnt that difficult, but building it right can be tricky, especially where reliability is concerned.
Cost wise, they can be built for less than an assembled one, depending on what parts you use. It isnt difficult to go way over the cost of an assembled rifle either, sepending on what you put in it, and where you buy the parts.
As far as a "truck gun", I think the AR has been beat around in some of the roughest terrain any truck will ever go, and still does its job. The mini may be a good rifle, but it wont likely ever have the field experience of the AR rifle.