The trailer I think you'd make from the old mower shouldn't be a burden on any machine. 3/4 acre and gentle landscape shouldn't throw too many red flags either, although the target price on lawn tractors is so aggressive these days that I'd go as heavy as possible anyway [don't think your needs really call for a GT though].
Now the quandry, you say as fast a possible. If you have obstacles, the ZTR is the choice for cutting time. The RZTs didn't impress me, but they were made for lawns like yours. I didn't recall much advantage of the 50; it has the twin cylinder, larger tires, 4pt antiscalp...maybe that is enough difference; Gamble would know better than I.
My statements on RZTs aren't exactly fair anyway; I was actually comparing the ZForce to a true commercial machine at the same time I looked over the RZTs. Rumor around the web is 800 hours on the EZT integrated pump/motor [sorry if I'm propogating rumor here]. The IZT, like the ZForce, sure seems beefer [and gives me more of a "warm-fuzzy", but I haven't seen anything that diffinitively states longer life. I guess if I were saving for a mower in that ballpark, I'd hobble something by unit I'd afford the ZForce44; however, if I'm imagining your lawn right, I don't know of a bad choice.
Last note: All other things being equal, since a Zturn cuts mowing time, you'll have maybe 2/3 the hours on the engine in 10 years [actually every time you mow for that matter] as you would a LT.
