I have 2 - a 86 GTII with a 18HP twin briggs and a 90's GT6000 with a 20HP onan twin - both use 44" decks - as of now, the GTII is the runner/mower - the GT6000 is the parts donor, shame really since it is the high dollar tractor of its day.
They both have the 3 hi/3 low trannys - i changed the oil in the 86 ( they take 10w30 motor oil) when i rebuilt it.
I plan on getting a front plow blade and sleeve hitch.
Other then the inital PO's neglect, i really havent had any issues with the GTII after its rebuild - good solid strong tractor, hauls heavy stuff easily . might want to look for some better tread tires if doing plowing tho- the stockers dig in too much and spin.
One thing is its a HEAVY tractor - handful to move around. A few things id suggest - grease up the steering gears/spindles - has sort of a heavy steering to it and change the fuel line from the tank- the rubber lines degrade over the years - i replaced mine when i had it apart.