Alex, you may already know this but when buying a MHP that is not on a City or County permitted Public Water System and has its own drinking water source such as a groundwater well(s) then EPA law (that most States Env.

Ground water?

I suspect it is ground water.

You do not have to share your Phase I findings with the seller, so if it winds up being relatively free of potential soil and ground water pollution or contamination, you may be able to get a below market deal. 

It can be a LOT more than 10k depending on a million different factors that you don't know at this stage (for instance, many foundation issues are caused by ground water issues that could cost you tens on tens of thousands to fix--or might be un-fixable in some scenarios). 

Waterproofing a basement is only a barrier to ground water (if the soil gets saturated) and will not offer any significant protection against surface water (as in flooding) ...

In fact most land does not have water rights aside from ground water for inside domestic use only.Your other questions are very broad and are difficult to answer.

Personally, I don't like buying property with major fire damage, or properties that have groundwater intrusion issues that can't be easily fixed by downspout re-routing and/or grading. 

What would you do if your well went bad and the groundwater became contaminated?

You've been sharing groundwater with a neighbor for years.