Personally, if space wasn't an issue, I'd take your top design, replace one of the normal bearings with a pair of angular contact bearings, and put a lock nut onto the shaft to adjust play out the bearings. That way endfloat is adjustable, the normal bearing provides extra support for the belt tension, and it allows for smaller pulleys.

I got my bearing and locknut from www.simplybearings.co.uk (for locknuts, click bearings on the front page then scroll down).

As for thread cutting, sharp tool (I bought a HSS insert one from greenwood tools, but I see chronos/glanze have them listed now), slow speed, and remember don't disengage the leadscrew feed once you've made the first cut when doing metric! Reverse the lathe back to the start.