Hey peeps why all the negative vibes???
Ah! You used the M word! Forum members around the country will be hanging up sprigs of garlic over their keyboards to keep out the wicked spirits of MDF!
Now, I'm probably the only forum member (or, at least, the only forum member prepared to admit it) to have built and still use an MDF CNC router. Personally, the experience of using this close relative of cold-rolled cow dung has somewhat influenced my views, but the machine works. In fact, I've just popped into the warm to do a quick CAM update of a design I'm cutting on the machine this evening. For reference, my machine has a cutting area of something like 600x400mm. Over the three years I've been using it, it has gradually been sagging and warping its way into oblivion, and while it wasn't that great to start with, it's worse now! I've had to bolt square steel tubes along the sides of the bed to try to keep it somewhat flat but still end up machining spoil boards or spacers for any job that needs halfways decent accuracy. The Z platform is sagging and is now a few degrees off vertical, and I've fudged it by packing out the spindle supports so that the spindle is vertical, but I can't make significant depth cuts in multiple passes as the cuts don't line up accurately enough. At the moment I'm cutting 3.6mm ply with a 3mm cutter, and running at about 500mm/min. I can hear the laughing from here - that should be around 10x faster but the structure of the machine won't take it.

More to the point, though, and assuming that you can build a torsion box structure, maybe judiciously reinforced with steel, that is strong enough to do the job, you still need to think about the subtleties of the design. For example, using ballscrews or whatever. I'm using cheap threaded rod as leadscrews on my machine, and that limits me to 900mm/min max. That's because of the leadscrew critical speed, which is the rotational speed at which the screw starts whipping and shaking things to death. I've just looked up an online critical speed calculator which suggests that the critical speed of a 25mm ballscrew, 3m between bearings (which is about what you are looking at) is just under 300RPM. If you use 2510, that means about 3m/min max speed. That might be OK, but most new builders are surprised to find that things like ballscrew inertia are critical to machine performance as this limits acceleration, and that's a big ballscrew you're planning. There's a reason why big machines often use rack and pinion or ballscrew with rotating ballnuts. There are plenty of other non-obvious design decisions and trade-offs.

Personally, I'd be surprised if you can build a very successful machine of this size in MDF, but in any case bear in mind when you are buying the more expensive bits that you might want to use them on the Mk2 when you get dissatisfied with the Mk1! That's what I did with my own machine, and I'm halfway through building a bigger machine in steel at the moment. But I won't say don't bother with the MDF machine, but be prepared to be disappointed. However, as a hobby machine, MDF gives quite a lot of fun per pound. I've learnt a lot from mine but based on that experience, you are being pretty ambitious with yours.