If loader equipped, get 4WD. If lawn work, get turf treads. Check any access problem that could limit size.
A 20-horse will do everything a 40 will but take longer. If most work will be mowing the big decision will be front, mid, rear. I prefer front but few utility tractors are designed for them. Mid gives more stability and doesn't increase the rig's length but costs more and most are harder to service, install, remove.
Two fail-safe brands are Deere and Kubota. Chinese makes are half the cost but you take your chances. Kioti offers more features for less money and quality that seems somewhere in the middle.