VFDs are generally more cost effective. If you really want the DC route,
some good deals can be found at Surplus Center.

I've converted several machines to three phase/VFD and been really happy. If
you have room in your machine, increase the motor horsepower a bit. First
you never can have too much power, and motors only develop full power at top
RPM. I think they are all constant torque. Horsepower is torque*RPM, so you
have 1/2 the power at half speed.

As far as how they work internally, there's a little box that contains magic
smoke tightly packed in there. You don't have to worry about anything, as
long as you don't let the smoke out. VBG You want a 230 volt VFD and a
230 3 phase motor. Connect single phase 230 VAC to L1 and L2. Connect your
motor to T1 T2 T3. You can use the VFD keyboard for for all functions but I
find it far handier to connect switches for forward/stop/reverse and a pot
for speed.

Here's the best place to buy your VFD
http://www.driveswarehouse.com
You probably want the L200 series drive.