20mm rails are more than large enough for any DIY router.

If you go with the AC servos, you're going to want to use 10:1 planetary gearboxes, with as little backlash as you can find.

There are several issues with driving a gantry from the center.
1) Racking will be an issue, unless you build a very massive structure.
2) This method requires a table that's suspended at the ends only, which will be less rigid than a fully supported table.
Using two slaved motors is easier, and works fine.