The 231 is the late 1980s/early 1990s version of the 135, which is a highly regarded small utility tractor.
The Perkins diesel versions are the most common and most desireable.
A couple of issues to be aware of: Dry drum brakes and fully UN-synchronized, sliding-gear transmission.
I have a 2007 533, which is the 2007 version. Aside from a few upgrades, 50 more cubic inches and a few more HP, it is much the same. It's been a good mowing work horse aside from a broken lower lift pin at 245 hours, which AGCO refused to repair under warranty, due to elapsed time, and a few other incidentals.
Note: You will need chains for MI snow removal.