There is a thread on cnczone, probably the better part of a million pages long by now, but somewhere in the first 1000 pages are many attempts to get an epoxygranite thing that is much stronger than wood. After reading , i think i would just go andbuy a small secondhand /scrap cnc machine to convert.
I repair and occasionally maintain, a good few cnc machines - so if you are going to build one… .
ball guides and screws - I've replaced guides that had been red with rust and noisy for months while the machine turned out perfectly usable parts - if you can't afford the best available, buy cheap, they'll still work.
thrust bearings and rigid mounts are vital - backlash cannot be fixed in the code.
use the biggest and best servos and motors you can get
use a good spindle (the last one we had replaced cost just under £6000 exchanged - but you might want to use something cheaper )
don't spare the coolant - cnc does a lot of cutting in a hurry so keeping the swarf clear of the cutter is vital - buy a mop 'cos it is always on the floor