AUSaxon, I have a TC, not a TZ, but I just traded in a 5' Befco finish mower for a MMM on Father's Day. I know a finish mower is not the same, but let me relate the problems I had.
First off, the tractor will have enough HP to power the 5' mower, regardless of if it is a FM or a rotary cutter. Second, the tractor's 3pt will lift the weight of the cutting deck.
HOWEVER, the weight of the cutting deck is probably heavy enough that you will lift the front wheels of your tractor right off the ground /forums/images/graemlins/blush.gif Don't ask me how I know that /forums/images/graemlins/cool.gif
I had to put 100# of weight on the front end of my tractor to keep enough pressure on the front wheels to steer the machine. My property & landscaping are designed such that I cannot really mow with the FEL on the tractor. I have not checked the overall weight of the TZ, but I have to believe it is slightly lighter than the larger TC.
I think I would follow your dealer's advice and rent/borrow a 48" unit instead . . . unless you want to do wheelies and/or can mount some weight on the front of your machine. Also, I am making a wild presumption that you are on F_L_A_T ground and are not going to try to mow down 3" diameter saplings when I say you will have enough HP to power the rotary cutter. If you are just cutting down a field, and it is flat, and you can add some front weight, then you might want to borrow that 5' unit to see, but . . .