Park and home ownership are two different businesses. Just like MH and RV parks are different businesses. I suggest you propose to the seller putting the homes in a separate company/LLC and letting the seller keep running those as a tenant of yours since he never learned that rentals are a bad idea.
We got into this biz to AVOID experiences like being class D apartment landlords, yet this is what you are proposing for yourself. We like tenants who own their homes, vs. apartment renters who own nothing and will leave in the middle of the night.
If you buy them, value them at their wholesale value per NADAguides or comps. Homes built before 1990 have very little residual value, regardless of the rents. Homes built before 1980 may have NEGATIVE value if you have to pay to remove/recycle them.
Remember, lenders won't give you much, if any, credit for park-owned homes. So the seller had BETTER be willing to carry a note on those, and the terms had better be good enough to allow you to compensate for the expenses and vacancies created by P-oH's. Your expenses on the homes could easily be 50% of income.