Sketchup

hmmmmm
i think you could use sketchup for walls ect.
i wouldnt recomend using it for characters or objects though. my opinion is that it can be used for the very basics but i dont recomend it for a comercial game.
you can do as much in say...... unity as you can do with scetchup (not much)

I've worked with it a bit. You can do some mechanical stuff as well. But it works very differently than 3DS Max and the like and has some significant limitations. I'm not certain how you would be able to integrate into the workflow very easily.

If money is an issue you can use the software I assume you are aware of named Blender. It may take more work however its better in the long run...much better.