For the modeling there are things you just can’t avoid, but something people in studios do today when a model don’t need much attention is to do a quick UV unwrap (also decimation master) in zbrush, the uvs and wireframe won’t be the “best”, but if works for that model its fine of course the downside for it is that your models won’t be using their full potential, you just have to pick your fight.
don’t wanna to get too much in the BSP thing there are plenty of threads about it already, but bottom line is BSPs won’t give you many options (can’t control how it renders, collision, its lighting, they are slow, and so on) its great for prototyping but not generally good enough for the final product.
Now for the workflow modular is the way to go, it does not mean you will make everything modular, its always good to have some “Hero” models, but that depends on the level.
you like Elder Scrolls…
I really like the way they approach modular level design on Skyrim
*if you are just having fun with a small cabin you don’t need worry about making it modular this is for when you make something bigger down the road.
the guys from bethesda explain it here:
have fun 