The heater is of 1940s vintage and a big time fire and CO hazard, so now I have some new heaters en route from Amazon to replace all of the outdated heaters on the property. 

@Mindy JensenThe shingle brands that carry products like that are Certainteed Presidential, Tamko Vintage, GAF Grand Sequoia I am not aware of any other brands that do that.

This seems like a vintage property.  

If would suggest finding a contractor familiar with improving the building envelope (air sealing and insulation) for buildings the vintage of yours as applying techniques used for modern construction on older buildings that were not designed to be bottle tight, may require additional measures to avoid inadvertent - and undesirable - outcomes.

Funny story: I bought a vintage Norton sharpening stone for a $1 once at a yard sale.

Instead on Redfin use the filters to find Comps; properties that have Sold in the last 3 months, similar style, that are within half a mile of the subject property, that are about the same sq footage, about the same vintage, and about the same level of finish. 

I am rehabbing a small 1950's cape that I feel has a really good potential but like many capes of its vintage it has a pretty claustrophobic floor plan downstairs with many small rooms and 7' ceilings. 

I am looking for a 1980-2000 vintage asset in the south to southeast side of Houston, Texas.

I recently added a vintage home, a HUGE one, to my inventory........already have a very qualified tenant.

Those ARE real expenses that are going to effect your NOI and therefore cash flow.You can also ask the what the market cap rate is for similar properties of size and vintage and if the seller has any flexibility in the price.