I thought I might chime in.
" With that much grass, I think at a minimum I'm going to need a 7 or 8 foot cutter."
You may want to price a 8ft cutter. (I assume you are talking about Bush Hog type brush cutters) They get pricey. You may want to ask about HP requirements. I think @ 45 PTO hp you will be underpowered. If its just grass and REAL light brush your cutting you might think about a 9ft sickle bar mower. They require much less HP and good used ones run around $1200. A 6ft rotary cutter could be used for everything else. 50 acres is a lot of cutting!!
"But I also like the 4WD feature as I think it should be especially useful for snow removal and work around the pond area."
Other than the price, there is nothing a 2wd will do better than 4wd. They are much safer with a loader. I don't think you mentioned a loader, but I think you will want one some day. They are by far the most usefull attachment you can have. When your in 4wd you also have 4 wheel brakes, which is good while working around wet areas, like ponds.
Happy hunting, Bill