Actually buying a new tractor youll want to use what the manufacturer reccomends while under warranty , or can be voided- do what service they suggest ( take it in ) when they suggest it.
Most new motors today are weak, underpowered and full of plastic , personally id go after an older machine .
New machines arent problem free , so expect some warranty fixes , older machines already have the bugs worked out.
Do some research , check into warranties, id go for a motor with oil pressure/oil filter , and stay away from box stores.