I use that Makita in a 2’x3′ configuration with Camar0’s and the Middle_Z_Rigid mods http://www.thingiverse.com/make:169149. I don’t know about the 1.5m/5′ length rigidity though. I agree with Vicious1 that if you have not, you should try the stock MPCNC with these mods.
Also, I don’t see any good reason to go with a geared stepper at all. My NEMA 17 steppers (datasheet enclosed) are very slightly beefier than the standard ones and they have no problem moving the 3.1lb Makita and the extra bearings in Camar0’s mod. They also get barely warmer than room temperature. I think it is a good idea to use the beefier DRV8825 drivers though. I have a small 30mm or so fan blowing across the drivers and they seem to run cool.
I recall that Vicious1’s “parallel processing” printer configuration has extra X or Y sliders and It seems to run fine at high speed. Remember that you will be milling at a fairly low feed rate so moving around the extra mass of the Makita will not translate to that much force on the steppers. Does this make sense? What do you think?