Is it bad that I like to make/edit my own models?
But seriously, regarding things I cannot model (cars, guns, airplanes), I just download objs from whenever I can find them and import into Blender.
I'm still not used to CSP, tho I have downloaded some mods there. (never heard of Acon either, so I'll give that a look)
Depending on the mod, I'll let CSP/Blender trace the form/lines of the obj, then draw in the gaps and custom details to make it look more like my style. If the obj has a texture/UV map I want, I'll mostly use Blender to utilize it (via screenshot, render). It's all a matter of me being too comfy with Blender, and there being more than one workflow to approach things, which is why I love it.
I used Sketchup before in the past, but can't remember why I dropped it.
Bottom line, my preference: Blender, OBJ files, adjustable/modular if possible (but not a requirement), many places online to find it/distribute (can't import objs/mods from CSP store directly into Blender as far as I know, so distributing on more than one place I think would be beneficial)
Hi-low poly? doesn't matter to me, I can work with either to suit the style I'm going for.