All help is appreciated and I feel really bad because my other tenants go to work and have children that go to school and they are not able to sleep properly because of the loud music. 

So, there is even more risk.The things that you can do, include being properly capitalized, not be greedy, (in whichever way that may be defined in the county you are buying), know solicitation laws, know Dodd-Frank, the Consumer Protection Act, the S.A.F.E.

We typically pay somewhere in the range of $3500 for a new, basic furnance, including install, if there are no other modifications to the system required.  

Can anyone recommend a reliable plumber or handyman who could install it for me?

Get an attorney and get these losers out then hire a property manager to properly vet any future tenants! 

To add value to the property, we undertook extensive renovations including the removal of over 40 yards of trash, installation of a new septic system and wall-hung boiler, and a new well pump.

@Charles Perkins is absolutely correct about "collecting all receipts, properly tracking all income and expenses and your basis for all assets properly documented and recorded."

To add value to the property, we undertook extensive renovations including the removal of over 40 yards of trash, installation of a new septic system and wall-hung boiler, and a new well pump.

Drywall installation and finishing is harder than most people think and whoever did that job - see the tape showing - didn't think too hard about it. 

If it costs $100,000 to install an ADU and it rents for $1,000 a month, then it may be worth it.