I suggest something in the 30 to 35 HP range.
I am ASSUMING that this is for maintenance, mowing, moving manure, hay, etc.
If you have a center aisle barn and want to drive through with a dump cart, measure your aisle width before tractor shopping.
There are CUTS (Compact Utility Tractors) and there are NCUTS (Narrow CUTS, some folk rearrange the acronym).
If the aisle is only 8ft wide you might not want to drive through, if 12 ft, perhaps.
Needless to say, turn ALL the horses out BEFORE bringing the tractor through for morning stall cleaning (-:
It sounds silly, but I do know barns where they leave boarders' horses in the stalls because someone is coming to ride at mid morning - when they show up at 10:30 the muck trailer and tractor are still in the aisle, more or less in the cross ties (-: