I have a feeling your motors are fine, and just what you want for a router build, but I'd rethink the idea of putting that mill spindle on it. Unless you're really building a mill for metalworking, in which case you're likely going to have to rework the whole design to be much stouter, it just seems out of scale for your machine. Wood cutting is best done at higher RPMs, so the low range of that spindle won't do you much good. And it's way too heavy, as you're starting to realize. If you want a more heavy-duty spindle than the 2.2kw Chinese ones, think about using a Colombo, HSD or Perske instead. These are beefier, but still have ER spindles that are easier to change tools in than R8, and don't weigh more than about 35 lbs. This will still require some counter-weighting to keep it from dropping to the table when you power down (I like gas struts for this) but not nearly what it would take to deal with that mill spindle.