I'd be interested in learning about your work flow to. I have always just saved out as a dwg (from FormZ), then imported into Max. I assume you are just linking the dwg?
I would also look at Final Render. They just released Service Pack 3, which has some great updates. A new version should be out sometime that is supposed to be standalone also. No idea which programs they'll choose to have it work with. Right now, I think it's Max/Viz, Cinema and Maya.
Keep in mind that any interior animations will take a TON of time to render, Vray, Final Render, Brazil, or anything using GI. 10 minutes a frame is fast, easily going straight up from there.
Also, make sure your firm is charging a ton for this service. It's bad enough that the architecture profession let's itself get ripped off by clients all the time. This is a valuable skill and service, and you should really be charging more than you do for architecture to do it (simply because architects charge so little). I am speaking as an architect, by the way, that does mostly professional renderings.