Well that depends upon the engine.
Walker generally fit top shelf engines so 1800 hours would not worry me on a Walker
Just replaced a Triad with a CH750.
Engine cost was $ 960 from Small Engine Supplies and they have plenty left at this price.
They were originally for a Cub Car but it will fit as Walkers drive off a stub shaft bolted to the flywheel.
Commercials are designed to do 1000+ h/year.
I can not see any problems with your plan.
1 Acre a year will be a doodle just be careful with the roughness of the grass as Walkers have very little n the way of ground clearance so little things like drainage channels will leave you high & dry.