You might look at
http://kronosrobotics.com/xcart/customer/home.php. The Dios chip is of the pic 18f family. They sell carrier board kits to make it easier to hook every thing up. They also have some app notes on a rs485 interface that may help with the communications issue. The chip is programed in basic with support for inline assembler if you need it. There are coprocessor functions in the language but, you would have to duplicate them on the robot controller. Still not sure you really need a coproc. The items you mentioned don't sound that intensive. As for the arm being back driven, worm gears are very good at preventing this. Both the van door and window motors are worm designs. We choose the window motor last year with additional reduction and had no problem with back driving. Our arm could reach the tall goal and place the ball when our drivers where on. I always prefer to handle problems with mechanical design instead of trying to correct deficiencies with programming.
Gary Deaver