You are in rarified air already. 

I am buying a 3 family property for rentals but the heating system is oil with only one boiler supplying the heat and hot water to each unit.

I also have a deal that I've run the numbers on a hot market where I can make 50k-100k profit after rehab for a flip (I have 3 comps to support this sold this month) or I can make 3k/month in rent.

Pools, hot tubs etc?

This is especially important in a hot market and if a home is a really good deal.

For mold remediation (air quality test in livable areas came back acceptable), sump pump/french drain, vapor barrier and some jack post installation, it’s going to cost me between 12k-16k.

You can also build an accessory dwelling unit (ADU) that's completely separate from the main single family home, but the permitting process in Arlington is a hot mess- my neighbor a couple houses down had his ADU permit take 13 months...so up to you on what's worth your time.

Not as hot of a market at some of the bigger cities, but it's not a small town either. 

These numbers are my opinion and have been compiled from Air DNA. 

I once had a guest come and stay from a camping trip in Yellowstone, and they sent me very nice messages about how nice having hot water and lots of towels and offered to pay a little extra for doing extra loads of laundry.