My first router was built from MDF. It was the JGRO design, freely available via a google search. Designed to be built with little more than hand tools, although I did make a few small detail improvements by 3D-printing some bits and pieces. Cutting area, from memory, must have been around 600x450mm. I can echo JazzCNC's comments - it meant a constant round of tweaking and adjustment, pretty much every time you used it. In the end, the gantry was sagging so much that I had to shim out the motor mounts to get the spindle anywhere near vertical. I did some useful work with it, but the idea of long-term accuracy and stability from something built from a material with the structural integrity of cold-rolled cow dung is a bit laughable. My Mk2 machine is welded steel - that was one lesson learnt!

I wouldn't ever say that you shouldn't build a machine from MDF, but be aware of the limitations. It has the great advantage that it can be cheap, so when you throw it away and replace it with something better, you haven't lost much! But I'm not sure that I would pay big money for someone else to build one, and the idea of it being rigid enough for 8x4 operation seems unlikely...