I don't know anything about Dalles, OR but I'm intimately familiar with the decaying towns left behind as the steel industry in the US went from world dominance to bankruptcy...With no population growth, no income growth, no job growth, RE appreciation doesn't factor in. 

I was hoping to get some advice/guidance on initial steps that should be taken when targeting an area in decay. 

They decay and fall apart at a much higher rate.

Weather and ordinary decay causes every landlord to spend money. 

It's the momentum that will counteract any uncertainty that you have.

Worried if we put even more ventilating in that it will counteract the heat being produced from the ductless units.

The overall character of the area is definitely a large factor as it will determine the type of tenants living there/demographics: Yoga/Orange Theory studios, Whole Foods, Starbucks, micro breweries, wine and cheese shops, public parks, bike paths, green spaces, public transportation, good schools, nice restaurants, cranes/new construction, low crime rate, high prices all indicate a Class A area whereas boarded up buildings, lots of Payday lenders/check cashing stores, dollar stores, fast food, liquor stores, homeless shelters, army recruiting centers, addiction recovery centers, industrial areas, dumps, shoes hanging from power lines, junked cars lining the streets, high crime, blight, abandoned buildings, urban decay, burnt out buildings, low prices, low rent, etc. would indicate a Class D area.It's really pretty subjective though, I'm not sure there are universally accepted definitions but rather just people's opinions.

My plan is to purchase a 2-4 unit property in either Manchester, NH or Nashua, NH and follow the BRRRR strategy guidelines using either hard money or private money to help fund the down payment (or the full purchase price depending on the state of decay).

Coupled with the expectations of the modern buyer is the view that anyone can show them the properties so I think it applies more to newer agents who don't have the systems and experience to counteract the new "knowledge" of the HGTV buyer.

Also if most of the houses in the area are around the same size and are rented/bought then I don't really see it as a problem.Plus it has a big yard, screened in porch and a two car garage so those are all nice bonuses to maybe counteract the small house size.