With that much grass you should be considering the largest deck that you can get that will fit everywhere you need it. It sounds like you have the flexibility of landscaping around your mower so you can set it up to get your mower where you need to. 61"-72" deck would be ideal as your mowing time with a 48" or 52" would take much longer than you may like. Also, it would be wise to spend a good amount of time having your yard smoothed out so that you can mow at fast speeds without being tossed around. A little bit of planning will go a long way with making your mow fast and easy. At that point a mower with 12cc pumps that is capable of at least 10mph will get you going and maximizing your time. Scag Cheetah, Exmark Lazer Z or Ferris F320Z would be great choices. Be prepared to spend upwards of $10,000 but all of the machines should last you 10 plus years if taken care of.