i Always heard that the machine cat made between the 225 and the 235 : the 231 was not that good, bad of both worlds , so consider an other option if you have that.... goodluck!!!
The 231 is a 225 with the smoke screw turned up on a 235 under carriage which if badly worn would indicate a lot of hours on it. If you want production from an old piece of constructon equipment be prepared for some expensive repairs
Both are in the 18K $ range