Review title commitment and survey to make sure to deed restriction and/or HOA in place that would stop you from doing 4 plex2) Meet up with City/County planning office to make sure 4 plex can be build and lot has enough parking spot . 3) Talk to an architect to get an initial foot print, layout and foundation type (i.e. pier beam or post tension slab) 4) Bid out to get estimate for both soft cost (survey + construction drawings + Permit) and hard cost (cost of building)  Without #3 you really cant get hard estimate Problem is that you may not enough time to hash out #2-4.. prior to closing the land .. 

The home had severe structural/foundation issues which were not disclosed in that pre-listing inspection report that was sitting on the kitchen counter during our buyer's inspection.

I am talking about placing a factory built home on a permanent foundation with that home built to the same national standards as used for traditional stick built homes. 

But you have the 30 ft pilings... special foundations definitely drive up the construction costs significantly.I'd keep shopping like @Jon Klaus said, @Anand S. and compare the line items a bit if the GC's break them out.

As for mobile homes, if they have no steel girders under them, called stick built, they do gain value if set on a foundation not in a park.

What seems to be better deal a high cash flow of $670 in C class area with some minor repairs, possible major repairs in 5 years. like roof, maybe foundation issues.

Read a book on how to manage dealsThat will give you a foundational understanding and then you can refine your knowledge based on how you want to invest.

I'm actually looking for large value ad stuff where the house is a pretty light rehab EXCEPT for major structural or foundation items, because most of those kinds of items now don't scare me away and I can differentiate and compete with less people when finding them.It takes baby steps though, if that makes sense. 

I am newer to property management thanks for the reply to the post.The problem I am having is no contractor can tell me how water is getting into my garage other than its coming in from the foundation but where?

He essentially said that the existing perimeter foundation is not enough to meet the requirements of chapter A3.