I tend to favor tractors in the 45/50 hp range for my own personal use. (I'm taking care of my 45 acres w/47hp) I own tractors of 47, 61, 70, 85, 85, 95, 95, and 95 hp. (plus 2 "non-runners", antiques in need of restoration, both 32hp IF running) All but the smallest are used for large scale commercial mowing. From my experience (I'm 61 and have spent most of my life around tractors) larger tractors require no more general maintenance than smaller ones (per hour of use) . Granted, the parts may be bigger, but a tractor is a tractor is a tractor. Especially so, since you've already got a 6' mower that's no doubt a CAT I hitch, I'd stay with a CAT I equipped tractor, that being less than 50 hp in all but a very few instances. (Smaller CAT II tractors can be used in conjunction with CAT I implements) Too big OR too small can be a problem. I'd rather err on the side of slightly bigger than absolutely required than ending up with one that's too small. For MY PERSONAL USE, I find anything less than 40 hp to be more or less useless for anything other than a lawn mower. Not everyone shares in that opinion.
Used DOES NOT mean worn out by any stretch. My 47 hp tractor is 38 years old and is EASILY my most reliable. I've got a new one, less than 3 months old, that has been back to the dealer 2 times already. (tranny issues)
Take a serious look at your mechanical aptitude before jumping into any tractor ownership. (or financial ability to pay for repairs and maintenance) Warranties only last so long, even with the very best deals.
In ALL cases, my first suggestion is to establish a budget. How much are you willing to spend? THEN fill in the blanks. (ie tranny type, cab, in many cases the brand, ect)