I have an Infocus 4800, permenently installed at my bar with a 5'x8' screen. Bought it at Sam's Club with a 3'x6' roll up screen for about $1000. Where I have it mounted I have a 20' ceiling, and the screen is mounted on the wall about 8' high. I use a Vocopro video mixer (
http://www.vocopro.com/frm_product.html) that I use with a video camera and my Karaoke set up.
I suppose you could use it as a mobile set up if you use a roll up screen with a tripod mount or something, and the room and distance to set up the projector.