The engineering report covers plumbing as well

The main thing you should be looking at is septic( if any), electrical, plumbing and structure( water damage).

I have plumbing and electrical covered, but would love to find a GC or flooring/drywall/insulation contractors.

It cost me roughly $13,000 to build it, which included power and plumbing.

I need some plumbing, drywall and paint done. 

For example, you start replacing faucets and discover that parts of the plumbing are outdated or not up to code.

Inspection found old cast iron plumbing leaks, major water damage from a previously leaky roof, multiple electrical and gas code violations and termite damage.  

working on our first BRRRR project and this could be a good time to replace plumbing.

Often that means all systems, ie, electric, plumbing, mechanical, etc must be inspected and approved with the current code regs.It's best to know BEFORE you buy or try to wholesale what exactly the powers-that-be will require to un-condemn and give the property a c/o so it can be inhabited again.

Obviously, it may be difficult/expensive to install plumbing and gas, but it may make sense depending on the location.