I'd add the following:
Forget about machining metal, especially with a 12" Z. There's simply too much cantilevering in order for you to machine something as hard as steel. If you set your goals on softer materials like foam and wood, you may be OK.
An option you might consider if you go with a 12" tall gantry: build a secondary table that sits 6" above the main table. This will bring your work 6" below your gantry instead of 12". This will minimize flex in the Z axis for work that is shorter than 6".
Good luck!
Carlo |