Identify the uses. Identify the tools/attachments you'll need, as these pretty much dictate how much tractor you need. Loader capacity, if loader work is high on the list (and even if it's not and you know you'll need to be moving certain amounts of weight, if you don't have the capacity then that task is off-limit). PTO capacity, especially if needing to run big mowers. As folks have stated, running with a smaller machine can generally get the job done, but it'll take longer. I was only running a Kubota
B7800 for many years before getting my NX. The
B7800 has done a TON of work, and though it took longer to do it I was better off that I took that time because I wasn't sure of exactly how I was going to work my property: if I'd have had bigger equipment I might have done stuff that I'd have regretted. Also, don't buy for doing 100%; think about renting bigger power for the occasional odd-job: I rent excavators; I do stuff that would be taxing for anything but the largest backhoes, in which case having a backhoe woudn't be fiscally prudent for me.
As Threepoint suggests, think about a cab: I think that he might have helped influence my decision (for which I'll forever be thankful). AC in the heat, heat in the cold. I've been able to work when otherwise I probably wouldn't have: and keep in mind that sometimes conditions demand that you work whether you want to or not! Cabs also help shield you from insects and brush: I have a LOT of invasive blackberries, the cab allows me to go after them without fear of having my flesh ripped to shreds (I have hundreds of hours battling them on my open-station
B7800, so I know, ahem, first-hand). I also have a fair amount of wooded property and have to watch really carefully as I operate around my trees.
Larger equipment also means costs for attachments and repairs are larger. No real clear "win," all a matter of determining where to compromise.
I'm fortunate because I have two tractors, a small one, the
B7800, and the larger one, the NX5510. I never plan on giving up the
B7800 as it is extremely versatile: it is the ultimate zombie apocalypse vehicle
