I have had an STX375 for 12 years.  Previous owner had Cummins install 450 software, which is the hp I needed.  The Cummins in this tractor has a 140,xxx engine serial number.  If you find one with this serial number, make sure the engine has been rebuilt or replaced.  These had a two-piece piston and is a matter of time before it will send a rod out the side of the block.  I renuold mine a few years ago and installed the updated, one-piece pistons, along with updated connecting rods.  I have replaced all tracks a couple of years ago and cost me over $30,000.  They may be closer to $40,000, but depends on the brand you price.  I had bogie wheel seal issues the first few years, but may have to replace one a year.  I have the tools to do it, so it’s not a big job.  Keep transmission oil changed often and use Hytran oil.  Change bogie and idler oils every 1000 hours.  Install Kile clear hub covers, if they are not already installed.  The transmission calibration in the older STX models will only calibrate the master clutch.  Even when calibrated , these may shift hard between some gears.  I don’t know when it changed, but the newer models allow calibration of the clutch on each shaft in the transmission.