I can answer your questions from firsthand experience. Look at my list of equipment.
I have no trouble running my 6 ft KK tiller with my 3240. The only time I really notice it is when tilling deep in previously tilled ground. If that sounds strange, let me explain.
In previously tilled ground, I can absolutely bury the tiller. My little 3240 will grunt a bit when I do that. I just slow down a little.
In normal tilling, 6 or 8 inches in good soil, I can go about 1.5 mph.
I slow down on hard stuff because the tiller will just bounce. You'll figure it out quick.
I also run a 6 ft flail. Same thing, no problems.
As far as the box blade goes, you will never run out of power unless you want to run a race.
That being said, the only time my BB has ever stopped my tractor is when I bury the scarifers in hard soil. Then the tractor will just dig itself a hole.
But, If you go deep enough, even one ripper will stop a tractor.
One other thing,
the 3240 is rated to pull 2 12 inch bottoms or 1 16 inch bottom breaking plow. I pull two 14's and I think I pull it well, over 4 MPH on some instances in normal soil.
What I'm saying with all of this is that the recommended (by the dealer) attachments are well suited to the 3240.
