I'll let people in snowier climates respond on the plowing, but I will tell you that traction is often the limiting factor in these smaller tractors.
I had a 50" box blade on my 2920 (same as 2620 but more HP) and it was great. If full of material, the tractor would run out of traction pretty quick, so I couldn't imagine using a bigger box on that tractor.
How firm is the lower part of the driveway? If it's compacted well, have you though about just having a load of crusher run delivered, and use it to smooth over the existing mess and level things out? I'd hate to disturb a stable compacted driveway with a rocky base, seems like it would be asking for trouble and could be a lot of work (and in the end you'd still have to bring in crusher run anyhow). If you can solve the problem with a 4" spread of crusher run groomed with a box blade, maybe that's a good way to go.
Around here, the problem is soft soil and clay, so we have to build up a base of large stone then top off with crusher run. Sometimes even put down cloth under the base rock to keep it from sinking. So half the battle is establishing a firm base. If you already have that, don't screw it up!