That's actually a great opportunity to add an air/water/vapor control layer, too.One option for the heating would be to add mini-splits to each apartment.

Would you spend the money on tankless boiler/water heaters?

Since you’re so far over I’d leave it alone barring any water stains. 

The rehab budget got blown completely out of the water by "surprises" but that's half of the fun of any good learning experience, isn't it?

The red stain is probably iron from the water.

Is the park on city water & sewer?

Dora over to Sanford/Lake Mary which traverses some ecologically fragile lands with very well thought out design (elevated sections for wildlife underpasses, a lengthy span over the Wekiva River to have no pilings in the water at all, etc).  

BP familia,I have 8 acres of land (inherited near Smith Mountain Lake VA) with an old MH on it with a well and septic tank (i maybe able to hook up to city water $$).

What would be a good way to understand the average electricity bill, water bill etc?

A capex budget needs to include but not limited to: roof 25 year life span, floors 10 year life span, appliances and hot water heater 12 year life span, hvac 20 year life span, bath and kitchen remodel 20 year life span.