For income property, you can get good comparables based on the rents paid and costs of the property - if the seller is using an agent that agent should be able to provide those.A lot of these costs may depend on your location. 

Get bids from a few contractors, and make sure they break out materials and labor (you want to always compare apples to apples)NEVER pay the full amount of the job upfront.

Sub2 on a heloc is probably trickier since they can be called even easier than a traditional mtg, aside from the due on sale clause.

Traditional financing is probably out because of debt-to-income ratio.Anyone know the pitfalls of getting a HELOC in New York, or have any preferred lenders?

You can always compare with what you're getting from local banks.

Aside from what Roland mentioned about the traditional path of selling, sometimes the seller is another investor who just wants to unload their properties, other times they are adult kids who have inherited unwanted properties to not have to deal with the costs of it.

Like, should I lower the ARV by e.g. 10% as compared to a new house?

My question is does it make sense to hold a single family in a market that the rentals are low compared to the sell price and focus on the long term value increasing or should I sell and have a clean start in a new state?

What are some of the challenges that you have faced in the past when educating investors that may be more inclined or versed in the traditional real estate asset classes, Such as SFR, MFR, etc... 

Understand the fees involved and calculate the total cost for an entire year of management so you can compare the different managers.