Quoted:
Lawn mower partnerships never end well
THIS
.5 acre and getting a rider. You are going to need a push mower to trim around and by the time you are done with that you may as well mow the whole damn thing.
If you are dead set on a rider, buy a quality mower from an actual dealer.
Cub Cadet and JD from box stores are junk.
Go to a tractor dealer and buy the lawn tractor. Yes, you will pay more.
Another option, is Sears - I can only relate personal experience. Had a Craftsman rider for 15 years - no major break downs only change oil, belts, and sharpen blades. It was used to mow 3-4 acre lot and push some snow in the winter.
Put the entire agreement with your neighbors on paper.
Include who uses when, who pays for maintenance, who stores it and how do the others get access, what happens when someone moves, how are major repairs paid for, what happens if you sell it, etc.
Really, thoug, a half acre isn't worth the hassle