27 HP will be fine.
What is the most important when buying a mower:
1. Warranty: you want a mower with the best warranty
2. Dealer support: you want a dealer that is close by and treats you good.
3. Mower: make sure the mower will have good hydrostatic transmissions. Atleast ZT-3100. The EZT 2800 will probably be fine, but it is for residential use and you are cutting in slopes and cutting 3 acres.
Scag make very good mowers. Snapper is fine, but I heard they are made a little cheaper, hence the lower price. If you want a step up from Snapper, check out Ferris if you can. Same company (Briggs), but better. All in all, it is mostly a personnal question a part from the above. You have to choose which one is the most comfortable for you and the best on slopes for you.