Never seen 2x1 used as a frame before, the base would ideally be made of concreet. Not 4 posts into soil.
At least a 4 " thick good strong mix onto top soil.
MDF will absoreb water and look like flaky pastry in no time.
Its more hydroscopic than normal timber.
But i`m only going on a few I have built.
Not sure about my concrete laying ability would still consider it, if its cheaper than doing a wooden base then it seems the right thing to do
Only snag is I have a drain at bottom of the garden! will take some snaps later see what u think