Bart, there's more to it than just grabbing some bearings and metal rod. Bearings need tight tolerances to run true. A press fit for bearings are generally between .0003 and .0005. You're not going to find those tolerances at the metal rack of your local borg. You might get away with some pillow blocks and a piece of rod for a small spindle lathe, but a large chunk of out of balance wood is going to raise havoc with a sloppy bearing fit and cause you nothing but grief. That puts you right back to machining issue and cost.
Look at the article on the shop built lathe Greg showed. The builder, who obviously has a bit of fabrication knowledge, used a reclaimed Rockwell headstock. There's a reason for that, it's just not cost effective to try and make your own unless you have the proper machines to do the machining on hand.
Your 3hp, 3ph unisaw motor would have plenty of power, but it has a proprietary mounting system. You'll have to engineer a mounting system for it, which is ok, but consider this. If the motor goes bad in the future then you'll either have to find another uni motor or rework the existing mount. Used 3ph motors are cheap, check junk yards and scrap dealers in your area. I paid $20 for a 2hp 3ph motor new in the box. They go cheap, basically for scrap prices because there's no market for used 3ph motors.
A 2-3hp VFD will run you around $145-$185 for a FM50. You can scrounge a cheaper one, but it's buyer beware... Controls can run you anywhere from a few bucks for cheap radio shack toggle switches to stupid prices for off the shelf commercial solutions.
Steel can be had fairly cheap from scrap dealers and junk yards. You'd be amazed what industry throws away.
I acquired and referbed a Rockwell/Delta 1460 with a 2hp, 3ph motor and VFD for right at $200. I had some of the parts on hand like the tubing for the base and an old base, wire, wood for the box and several of the switches which kept the cost down. I think you could reasonably scrounge all the parts from scratch for a lathe like the one in Greg's post for around $350 to $500 (with a new VFD) with some careful scrounging.
Mike