Jermey,
The reason I said go with an analog input for spindle is so you can also have "Manual" external pots for speed control at the machine (for overrides). If you dont plan to use a Pendant, OR, manual Spindle speed override pots then there is no issue with going to the modbus direct write of the Fx. BUT, if you want manual override then you will have to go in with a card, otherwise they will conflict and cause you probs/errs.
Motor Size: If you going to do high speed CUTTING, that would also include small frequent direction changes (like 3D work, or complex segment 2D work). I would consider more power, I am NOT a mechanical engineer (thus the reason I use the sizing software).
I guess you could try the 1kw's and if your not happy, you could sell them to me, or Ebay............
You are gonna play hell with a heavy gantry if you truely plan on cuttin at 1000ipm, that will be ALOT of inertia to change, Honestly, I think when you set up the ADC motor Accell curves (recommend double s), that for them not to clip your accel/decel rate will be longer than you think. Also, your gonna Drop that energy as Back EMF you are probaly gonna have to buy and extra disapating resistor for the dynamic brake. If you moving that fast, with that much wieght, you gonna be dumping a lot of heat of of the that same resistor so heat control will be a consideration.
You where saying that you might use the 1kw's but gear them down, to increase the torque, you gonna loose the speed.
at 3k top rpm, with lets say 5tpi screws your looking at 600ipm and that is without gearing. So if you then drop the ratio for more torque you will drop that speed further.
scott