Steve, the TC45DA can handle a 5' to 7' rotary cutter with ease. It just depends on your application. I have a 5' cutter so I can back it between trees and I know if the tractor fits between two trees, the mower behind is not going to hang up. I would say the 6' cutter is most common on the TC45DA, but some with a lot of clear area would prefer the 7' cutter. You could go to 8' on that tractor, but hold your breath on price, especially since many 8' cutters have two gearboxes and two blades. They are excellent because they are much shorter than a single blade design, but they are expensive.
If you think you want a posthole digger, why not rent one first and find out what they are like. You can buy a nice PHD with a downpressure kit and they are definitely nice, but how many holes do you plan to dig? Could your rent a skidsteer with a front auger? I own half-shares in a PHD with one of my neighbors and it works out well. We both saved money and we don't do enough hole drilling to keep that auger busy. It just hangs unused most of the time.