I was thinking you would use a timing belt as that gets an accurate ratio and negligible backlash. If you can afford a servo that will of course be excellent.

9000 rpm is a bit much isn't it?! Bear in mind that tapered roller bearings have much lower speed ratings than angular contact bearings, so if you want a speed above their rating you'll have to use angular contact. Ratings are here:

http://www.skf.com/skf/productcatalo...e=3&startnum=7

The reference speed is with grease and the limiting speed is with oil and optimal cooling, so best to be below that. Tapered roller bearings is likely the cheapest and strongest option with a pair nearest the chuck and one deep groove bearing at the other end of the spindle and not fixed axially to ensure thermal expansion doesn't increase the pre-load on the other bearings...

Driven ballnut - like I did for my router?

http://www.mycncuk.com/forums/showth...t-design-ideas

When I convert my lathe I'll use a rotating ballnut for the leadscrew so I can still have a handle on the saddle to use it manually. Also saves working out mounts for the ballscrew to fit on the annoying curved surfaces of the bed castings.