I had to repeatedly travel back to the house over and over for meetings with a specific firm of plumbers that the HOA insisted come there with the brief to persuade me to believe I had pipes leaking under my slab foundation - and push me to pay them for extensive unnecessary works.The previous (independent) pumpers and the engineer had already explored all this and confirmed no such issues The Board started inviting me to take part in the morning meetings they set up monthly to argue with homeowners who have broken rules. 

My company was hired to provide all the carpentry services (foundation form work, framing, cabinetry, trim work, etc.).

Answers to these questions can help build the foundation to finding a solution to your problem.

Also, ask for rent rolls, dates of rents paid, maint history, look at permit history on property, foundation, inspection, etc..Terms of lease are important so review leases thoroughly.

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.