OP,
At 20 acres a year, how many year's worth of use do you consider acceptable?
I doubt a cheap one from Tractor Supply (or similar) will give you 20-30 years of use. Probably 10? Factor in the cost per year and decide if that's ok with you?
It's only "unbroken ground" the FIRST time you till it. After that, your tiller won't take as much of a beating.
I've always thought of the King Kutter's and similar as "homeowner use" and home garden use type applications.