A spindle option for you:
http://www.a2mc-cnc.com/Spindles.html
We use the Elte 3HP @ 18,000 RPM and have been quite happy with it. We also have an auto-mister, which is a very nice feature for cutting aluminum. We cut up to 1/4" thick plate on our setup - multiple passes of course.
AXYZ Router (
http://www.axyz.com/) might even quote you an entire Z assembly + spindle. They build complete CNCs based on the A2MC controller, and do some custom work as well. Maybe they can give you a pre-fabbed Z assembly + spindle, and just let you figure out how to mod it to mount it on your X/Y gantry. If you go this route, again I'd highly recommend the mister.
As an aside, here are some options for stepper controllers if you don't get yours working, or if the old ones don't support ModBus:
http://www.a2mc-cnc.com/motors_drives.html
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