The difference in accuracy between C7 and C5 is not that much. Over an 800mm length you're surely going to have more inaccuracy caused by deflection of the frame/tool and the resolution / step accuracy of the stepper motors to an extent. The piece that the OP said is about 1m long will need to be strong and rigidly supported at the ends to not deflect by +-2thou.

http://www.mycncuk.com/forums/showth...curacy-V-Price

The C7 grade screws I have all seem fine, though clearly I have not got the kit to measure them that accurately. Either way leadscrews with the equivalent to C5 should be cheaper than ballscrews.

I would definitely use a screw on both sides linked by a timing belt. You could use two motors but then it's more difficult to keep it square. A small amount of skew will stop you getting 0.1mm tolerance. Why do you want that tolerance for kitchen worktop? How are you going to measure upto 800mm long to the nearest 0.1mm?