Getting the residents to chip in

I am going to upgrade the older units in my park. They are for the most part, faded, have little or no skirting and look awful. I found a bunch of used metal roofing panels for skirting, paint at $4/gallon and a bunch of leftover vinyl shutters all on craigslist, all very cheap. I am moving in new and repo homes into my park that look great. I am slowly selling them with owner finance but the biggest objection is the junkers already there. I believe a little upgrading will go a long way even if I pay for some of it.

How would some of you charge the lot renters for this?

charge $10/mo. for 2 years?

offer discount if they help paint or chip in?

thanks in advance