1. I'd say use no smaller than 200oz motors, and a HobbyCNC or Xylotex as the minimum. Both sell complete kits with motors and drives. Bigger motors can get you more speed, but when you need bigger drives, the price starts going up fast.
2. As accurate as you build it. Accuracy can be tough to define. IT depends on what screws you use to drive it, how accurate (straight and parallel) the bearings are, how much flex does the machine have, .....
The JGRO is really not suitable for cutting metal, imo. It has too much flex due to the unsupported rails.
No one can tell you what size motors you'll need for a bigger machine, without having a lot of info about that machine. More than likely, if you buy Gecko drives now, you can use them later. But, I'd go with the low cost option mentioned above first, and sell it when you decide to upgrade.