Test drove an FD yesterday. Car was stumbling in low rpms and was back firing. Any idea on what caused this and how much it would be to fix?

 

Have it compression tested by a mechanic that knows how to do a proper compression test on a rotary before you do anything else.

Also check to see if the vacuum lines have been replaced with silcone lines or if they are original. If they're the original, they WILL need to be replaced. That could explain the stumble...the twin turbo system might be all screwed up