is he willing to seller finance? if so, what are the terms? if he doesn't want to maybe you could do a master lease option. and whats the price? How many park owned homes? and are all 50 sites vacant? or do they have a home that needs rehabbed? is it in a metro area of 100k plus so you can fill the vacant homes?
based on what you gave $200 x 19 x 12 x .5(smaller park w/ private utilities) x 10=228000
With private utilities...you have to ask yourself is the risk worth the reward. Just based on the information you gave, I would pass, but I don't like private utilities. Or if a park does have it, there has to be something really positive that makes the deal great. owner financing with no payment or low payment for a couple of years.
also plan for the treatment plant to go out. to be conservative that it may cost couple hundred thousand to replace it.
One last thing to think about on any deal is your exit strategy. With private utilities for both water and sewer, when you go to sell, your buyer list could be limited because many investors shy away from parks that have both water and sewer on private utilities