Right for whom? Sounds good for the seller, but not so good for the buyer.
Given the condition, sounds to be in the ballpark. 100 acres is plenty of use for a rotary cutter, IMO.
I saw a JD mx10 3pt in my area. I thought this would be a very good deal. JOHN DEERE MX-10 mower
Here is another: John Deere HX10 10' pull type mower
Dead Horse
The mx10 does have height control on the back. Manual on the 3 pt and hydraulic on the pull type. Older versions I have seen have the height control on the spindles for 3 pt. Newer 3 pt version has the adjustment on the mounting arm connecting to the mower. Is yours a semi mount? That would be 2 pt connection in the front and hydraulic lift in the rear. They still make that version as well.
What do you mean by slewing? Is that the ability to have the mower stay level when lifting the rear of the mower with hydraulic cylinder?