.OBJ and .FBX are somewhat different beasts. OBJs you're likely finding for free, and may thus have trouble getting textures to load. Often makers give away the OBJ but didn't texture it, or the textures only load properly in their software. An FBX should theoretically have the textures embedded inside it, but will likely have other weirdness and explosive tendencies once inside Poser.

OBJs can be saved to the Poser Library. Not sure about FBX as I avoid them, but I assume they can also be saved if they're just props. Yes, they will likely not look as good as Poser / DAZ props. You will thus want to build up a Poser Library of materials. I assume you want to render in SuperFly, in which case you want packs from makers such as GhostShip and Vinge Bagna.

As for buying rigged FBX figures not made for Poser, that's a bit of a false economy. Usually they are low-poly for games engines, anyway, and also tend to expensive. You'd do better spending the time learning about how to find (a very important skill) and then adapt an existing Poser figure or robot etc to your needs, with morphs etc.

It would be useful if you could say what your end goal is here: graphic novel, game, animation, and with what types of figures and in what genre.

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