If I did a couple vertical braces mid-span, you think I'd have to then add legs in the middle too?

You are dead right on the collets. I have an ER32 shank for my Bridgeport and was thinking ER16 was ER32 (powers of 2...damn computer geek :). I'll look at 2.2kW spindles.

Not sure I understand the Z clearance question... I'm planning on a beam gantry (if I have my terms correct), so wouldn't taller sides give me the clearance needed? Within reason of course.

Thanks,
Wallace