i wouldn't. everyone uses ply for a reason, and it ain't just cosmetic. I'd be concerned about pulling t-nuts through MDF. if you lay off the slimfast you'll be able to afford it.
my mate used mdf. and in these damp climates within 2 months he had ripped holds clean of the board, by shuttering ply, really cheap £12 for a sheet in the right palces, if you go to your timber merchant you can ask to look at it and just pick all the best sheets free from filler
or failing that, go round building sites scrounging packing ply.
do you need tnuts and bolts?, I have thousands lying around left over stock...
I've got a better suggestion. Take a short holiday to font, pick them up in the process. WIN-WIN!