Initial design advise wanted

[JAZZCNC]

With 70Kg + Gantry then I'd advise you go with servos if you want very best performance. Setup isn't so much an issue these days modern servo drives are much more forgiving. The Main downside is cost.?
With servos it's not just the Motor/Drive cost you have to factor in the that to get the very best out of them you'll need Motion controller that can provide the required Pulse frequency. Most of the cheaper USB Controllers like UCCNC etc cannot provide the required pulse rate to get the full speed and resolution out servos. Electronic gearing is required and this comes at the cost of resolution. However for most routers this is sacrifice that can be made without affecting machine performance. It just defeats the point of one main advantage of servos.

If your going with belts then 8Nm Stepper motor will be minimum and best run on Mains voltage drive to get best from it and give decent speed.
I've just built two Routers with 1250 length cut that runs Belt drive twin 2010 screws with 4.5Nm 34 motors running 72Vdc AM882 drives with Gantry aprox 45Kg giving 700mm width cut and they will happily run at 10,000 mm/min with acceleration 1200s/s.

Now with bigger 8nm motor you won't get those speeds with only 72Vdc but with Mains voltage drive then you will and probably more. Obviously the higher weight and inertia of the Gantry will play bigger part in lowering performance but still you'll get at least 8000mm/min with 10mm pitch screw which is more than enough for most routers at this level/size.

Like Clive mentions Don't cut corners on the Motion controller. Avoid USB it's too twitchy. Ethernet is by far more stable.
There are several suppliers of Ethernet Motion controllers depending on what control software your thinking to use. My prefered choice and only controller I'll use with Mach3 is from Cslabs. However the new Ethernet card and UCCNC Software from CncDrive are very serious challenge to Mach3 so worth close look. Not used this Combo yet so can't comment properly but I'm hearing good things from People who know there stuff and who's opinion I trust. I Will be testing shortly along with the Cheaper Stand alone controller.

Then you have the stand alone controllers like may have seen mentioned lately on Forum (but I don't advise you take Boyans lead and connect two drives to one Signal Output.) Not sure if the 4 axis version will allow Slaving but if your using belts this won't matter and could use 3 axis version.
Again Can't comment or give an informed opinion but I will shortly have one to torture so will post something at some point.

hope this helps.