hdb,
Just for entertainment and to see what others have, you might do a search for "hp/acre". I have 8 acres and only 18.5 hp, but I bought it mainly to mow and move snow and it works for me. If you want a 5'-6' rotary, then you will probably need 25 hp or more. I plan to purchase a rotary mower yet this summer and mine can only handle a 4', but I only plan to cut my field of about 4-5 acres down a few times a year...obviously it will take me longer than with a larger tractor with a larger rotary. I do pull a 3 pt. 5' rear finish mower by Woods which does a great job on my lawn of 2-3 acres. I also considered weight since I was mowing, mine is 1438 lbs. but some that post here are happy mowing with 2500 lb. tractors. You will also need to decide what kind of tires to get, I have turf tires that work great on the lawn and moving snow, 4x4 and hydro trans. help too!
The three major brands, New Holland, Kubota and John Deere are all pretty good, personally I found the New Holland TC18 that I bought to be more tractor for the money but that seems to vary by area. I think you should look at ALL brands that are in your area and give a great deal of consideration to the dealer, pricing, availability, etc.
Good Luck, JimBinMI
We boys and our toys!