Last week, we had an issue with the plumbing in a home we sold.The buyers moved in, used the water, and some areas of the home started flooding.We sent vendors out that same day to investigate and fix the issue.

I have heard developing an rv park one should budget $30k per space, which seems high to me but, all the 50 amp underground power, waste plumbing, possible lift stations, pads for rv's concrete use areas, pavement, landscaping, engineering, surveys, sitework etc, maybe?

A 1920s home with new plumbing, updated electric, and a new furnace might be in a better position than something from the 1990s.

A good rule - as long as the plumbing and electric has been updated, a well-built home from the mid-20th century is a better investment and will likely outlast a newer build.Credit to the respective author

There is no running water, the plumbing would need to be redone. 

HVAC duct work insulation is also deteriorated.No vapor barriers installed eitherSome fungi/mold growth notedPlumbingMultiple small leaks under sinks and from bathtub and toilets.House has Polybutylene plastic plumbing which is dangerousMain shut off valve is damagedElecticalSome small exposed wiring and bad splicing.Whole house has no GFCI's or grounding outletsInteriorMoisture spots on ceilingExteriorLose Vinyl SidingDoor Jams and Window trim have moisture damageDriveway need replacementRoofingMinor leaks noted in atticRecommended to replace roof covering, but not whole roofSome fungi growth noted in roofSo some of the question I am trying to tackle with this work is:How much should I budget for the main issues listed? 

I'm the landlord of a 3-unit building and wanted to know if I should be concerned about any potential plumbing issues (like freezing pipes) or other safety concerns.What steps should I take to protect the building, and are there any other things I should be worried about in this situation?

Because of their experience, I was able to get parts of the work covered under code upgrades for things that I did not build back exactly as they were before the claim (e.g. moving plumbing to expand a bathroom).At the end of the day, no one is going to care more about your property than you.

Without the sprinkler permit you can't pull an underground plumbing nor interior plumbing permit and if you're amending the plans that could hold up both of those permits, which could stop your build in its tracks, depending on when the mistake is caught. 

Hey @Devin Bost, I would highly recommend a few things to help with this. 1) Have systems and processes lined out for any emergency you can think of.2) A spreadsheet of subcontractors/ vendors that could handle the situation that arises (plumbing leak, electric out somewhere, etc.)3) Your cleaning team and handy person should be utilized for some things.4) Something like Breezeway can give you 24/7 support around your property with a dedicated phone number line.