I concur with the comments regarding park relationships and to not overbuild.
My mentor, who is highly successful with MH parks, said to try not to spend more than 5k for rehab, except in the rare case of a really upscale MH park. Use materials meant for mobile homes (which are much cheaper) - think, what will/might break if the house must be moved (drywall for example)? He also said he includes the lot rent in the rent to own monthly (i.e. $800 charged monthly, $300 lot rent = $500 actual cash flow). You're right that it wouldn't be attractive to have to pay rent for the home ON TOP of lot rent - they might as well buy or rent stick built.
If you crunch the numbers, the ROI / net CF versus what you might pay to buy the home is much higher than stick built. A great upside with very low capital needed; definitely buy low as you'll have trouble getting bank financing unless the house has been put on a permanent foundation (converted from a vehicle/personal property to real estate).
When determining sale price, it's less about the stick built comp sale prices in the area. You should consider what fully fixed mobile homes tend to go for in the area (versus new), the location of the lot/area (i.e. land appreciation and/or rental demand in the area) and the type of park (if applicable). Other possible factors is if it is specialized for vacation, assisted living, senior living, etc. (Think Florida for example). MH owners tend to care more about the affordability of the monthly cost and term (for rent to own) than the actual full sale price. The decision making for buyers tends to be different than for stick built. They can't easily get loans for previously owned homes, may not have good credit history or have had to start over due to bankruptcy or divorce, prefer to live using cash instead of credit or banks, want / need mobility, want some yard space, enjoy communal living, dislike urban living, want to save money or have limited income, want to live simply, etc... Always best to base your buying / marketing decisions not on what you might do/like but what your target market wants/does.
Think about the average monthly rent for comparable # bedrooms for (5-7)? years... what's that number? I bet it will more than cover what you paid to buy it and the cost of reasonable rehab.
