Last year I did a ton of research and demo'ing. I also mow about 3.5 acres of fairly rough ground. I found the 5k-6k price range to be the sweet spot between price and quality for this kind of duty. There seems to be too much corner-cutting to shave 1k off the price.
I nearly got a Gravely ZT HD 60 but ended up with a Husqvarna MZ 6128 because for 5K I got larger 23" tires, the 3400 series trans and a great dealer. However, I see that this year the specs are a little different with 22" tires and 3100 trans, so it may not have as much appeal for me this year. I do like it, though, and still could recommend it, but it doesn't have quite the bang/buck as last year.
Regarding price, remember that if you choose right, this mower will last maybe 15 years or more with good maintenance. You probably won't put more that maybe 30-35 hours per year on it, and anything in the 5-6k range should last 1000 hours or more easily. I'd go for the more durable transmission, BTW. They're not cheap to replace and get worked hard.
I wasn't sure between a 52" deck or 60", but I'm glad I got the 61. It does cut the time down, which is nice, but also the time put on the machine.
Given your options, I think I'd go for a Gravely ZT HD, but don't underestimate the value of a good dealer in the selection.