Wife got this over 10yrs ago from an employee that just couldn't stand his neighbor having a zero turn. It was in rough looking shape but ran fine.
Over the years I have done:
oil and filters of course
Battery
Air filters
belts
welded the mower deck with an $89 HF wire welder bought for the task
welded the muffler with the above wire welder
fuel pump and filters
hydrostatic filter and fluid. Tractor Supply Universal fluid works great
electric clutch under $100 direct from domestic maker
re-wired some of the safety switches and some other connections plus pulled the headlight wires.
PTO switch
Starter for about $60 but it didn't really need it I think
Thats about it, probably less than $500 into a 30yr old mower that starts and runs like a top. The original Kohler Command 16hp just keeps going.
I've learned a lot about old mowers tinkering with this one. I'm looking around yards as I drive about and there are a bunch of riders just sitting. I'm considering picking up some and getting them going then selling them. Craftsmans (red and gray), Husky's, even saw an old Wheel Horse. I bet most will get moving.