On brand new construction that minimizes the exposure to old mechanicals breaking. 

Mechanical parts wear out and anything that moves can just break from normal use. 

The big difference in costs are in the following areas; separate utilities for mechanical, electrical, and plumbing, most likely the need to upsize the water meter and meter lateral (Will need a DC Water and DDOT permit), fireproofing between units, and a sprinkler system in one or both units.

The mechanics behind the loan are exactly the same.

One inside they are generally qualified to check the mechanicals including electrical systems, breaker panels, junction boxes or lack thereof, knob and tube, outlets (condition and grounding) GFIs, etc; Plumbing systems check include hot water tank, faucets and drains; HVAC check; etc.

I am curious to know if anyone has used Roofstock as a mechanism to get started in real estate investing, or to expand their current portfolio?

Are you mechanically handy and will this be your job also?

Builders can put in a clause that states that if materials rise in cost more than XX%, they can increase their price to compensate.Whether they have some other mechanism built in to be able to cancel the contract themselves, depends on your contract.  

All major mechanicals replaced.

All the exterior had some rough patches, inside all mechanicals have been updated.