Hi Gilbert,
Fergusons seem to have a well earned reputation for being reliable, long life tractors, especially the Perkins diesels. Of course, I and others here may be less than completely impartial.
Besides the planetaries, there are a couple other things to check.
Pick up one of the lift arms, and see how much play there may be in the rockshaft at the lift housing - a little may mean just new bushings needed. A lot will mean replacing the lift cover. Both will require the services of good machine shop to press and line bore the bushings to fit the shaft. You may wish to replace the rockshaft if it is no longer round at the bearing surfaces. Yes, parts are readily available.
Check to see how fast the 3pt drifts down with the tractor off and the lift spool valve centered. The slower (or not at all), the better. Also check the hydraulic oil dipstick by the operators right heel - is it clean (changed recently)? milky (water in it)? less than full (leaking somewhere)?
These have a hydraulically activated PTO clutch. It isn't uncommon to have the PTO brake wear out, especially if they are used with high momentum loads (brush hog) without an overriding clutch. A worn brake will allow the shaft to turn even with the PTO off. Besides the safety issue while nothing is hooked to it, it may actually drive light loads (like a PTO rake when not on the ground)
You should check the front axle pivots - with the age of the tractor, they are likely to be pounded pretty badly. There are supposed to be brass bushings in them, but are often nearly gone, and both the pins and holes can be egg shaped (mine were). The back one is nearly impossible to see without a can of solvent, an inspection mirror and small flashlight, as it's an inch or so in front of the harmonic balancer. The front one can be checked by removing the front pin cap (two socket head cap screws). If you can get another tractor to pick the front of the 165 to see how much play is in the bushings, that would be an adequate test. If they are good, there should be none perceptible.
Also, while my MF doesn't have Multi-Power, there seems to be a long list of posts on here regarding M-P problems. I'd be more likely to knock off a few bucks for that potential problem than a straight gear tranny.
I hope this is what you are looking for, and works out for you.
Well Worn