I want to get a PDW and upgrade it but do not know what brand I should get. I do not really care about internals, but would there be much of a difference between the external quality/ durability between different brands? I am looking at the cheaper models. -Dboys, Matrix, Bravo, or any other brand I can find used locally. Would any brand be better than the other externally? Also would all of these accept a normal V2 gearbox?