Trying to replace the aged heating furnace in my property.

Challenges - We started with a rehab team that was screened and on board, but finding replacements and additional contractors was a challenge at times.

You should always ask for a "Reserve Study" to determine what the HOA is responsible for and how much it cost to make replacement or repairs to said items. 

They replaced and brought two water heaters up to code.

The 3 biggest components of your closing costs are you insurances... title insurance which is usually around $1200, and then your homeowners and flood insurance.

The “guts” of the house have been replaced in the past ~5 years in preparation to sell the property.

Also, some will do Replacement cost while others only do Actual Cash Value for how they payout on the loss.You should probably seek quotes from an independent agent.

When was the HVAC, hot water heater replaced?

But she had to go buy a new lock set to replace the one that was broken and had to be forcefully removed. 

I stick to tasks I am allowed to do (patch drywall, paint, replace fixtures, flooring, tile, and cabinets) and hire licensed contractors to do the rest.I've noticed only 1 out of 6 licensed contractors I've hired in the past (HVAC / plumber / electrician / roofers) actually pull a building permit for the work they've been hired to do.