Ugh. Unsupported rails. That will be good for extremely light duty at best. Slow feeds are your best bet. That whole structure will sag in the middle.
I agree that a slow feed rate is your friend, but cutting MDF shouldn't be a problem.
The MPCNC can mill aluminum with the right combination of bit, spindle, and feed rate.
! No longer available! No longer availableThe designer recommends a typical 2' x 2' working area, with tube supports for any axis over 4'
or go with the newer LowRider design for a large working area.
One technique mentioned on the MPCNC forums is to rotate the tubing 180 degrees from time to time so any sag induced by gravity is then reversed by gravity.
If that fails, replacement conduit is relatively cheap.
All things considered, it looks like a great way to get started with CNCs for under $500.
Scott