Using rendered 3D images for backgrounds has been done in other webcomics before, to varying degrees of success (Sadly, none spring to mind, but I've seen it before). Basically, low-poly isn't a problem so long as you shuffle up the camera angles once in a while. Don't want it to get bland.

Another technique is to use photographs for backgrounds... I know Hammerspace uses photos for its backgrounds, and that works fairly well. Of course, Hammerspace is kinda a sprite comic, so it only needs one angle. :-)

Webcomics are fun to do, trust me. :-) Just make sure you've got solid enough ideas to go with, you don't wanna run into a creative block down the line. Though it looks like you've got fairly good starter ideas. Should be interesting, just gotta develop stuff more.

And always remember, a large chunk of webcomiccing is the community aspect of it... If'n you want help on starting up a webcomic, you can ask me if you want.