Yep - a 3-phase motor was another idea that crossed my mind, but I did see a few posts on here about them being unsuitable for delivering high torque at low speeds and that they might overheat at low RPM. I found a UK website via ebay doing VFD's and 3-phase motors for about £150 for a 3/4 HP kit :
http://www.drivesdirect.co.uk/Produc...r_Packages.htm
I can't tell if the motors could be mounted with the spindle vertical though...
I was actually wanting to get the parts for the spindle from an X2 or X3 - it should be stiffer than the X1, and I was planning to connect the new motor via a belt drive. Looking at Littlemachineshop.com, I could get the X2's spindle box and a belt drive kit for $274 + VAT + duty - which isn't too insane given the strength of the pound.
Still, a variable speed router and a DIY pulley drive conversion would be a whole lot cheaper - the X2 spindle without bearings etc is £25 from Arc and they have a wide range of other little bits and bobs that might be useful...