I don't have any advice to offer since I know nothing about tractors, but that's a very nice present to give to your father.
John Deere. Mine is 20 years old. Change the oil, sharpen the blades.
One thing to note, both Cub Cadet and John Deere have two line s of equipment. The inexpensive line carried by the big box home improvement stores like Home Depot and Lowes. There is a higher quality line carried by the factory dealers.
Cub Cadet has a nice zero turn lawn mower for about $1,400.
Make sure the engine has a cast iron sleeve and it should last years.