A lot of these are ballpark, offhand estimates and I have yet to verify them.I also have an offer to buy the property as is for $170k.

You need this calculator on your phone or laptop so you can enter the estimated sales price, market rents, estimated expenses, and make a quick decision. 

A lot of these are ballpark, offhand estimates and I have yet to verify them.I also have an offer to buy the property as is for $170k.

Typically if I find a rental property, say a duplex worth 100k, I find a mortgage, at a rate at say 5-8%, with 20% down. estimating it to cost 1-3 points.

Actually, you were pretty close with your estimates :) We are looking to get ~$2 - $2.2m for our current house, and that would put us at ~800k - 1m in equity (after sales costs).

I understand there are multiple ways to estimate fair market value of a multifamily asset, but I am interested in hearing about your experiences with the ways you find most helpful. 

I know Brandon would say to analyze, analyze, analyze, and I will but I always get stuck on estimating the potential rehab cost.

The mortgage (plus taxes) is $700/mo and Zillow puts the rent estimate at $1,400.

If it is impossible for you to look at the property prior to purchasing, you MUST have trusted partners that can point out repairs/concerns and estimate (in dollar terms) the remediation plan.  

I got an initial HELOC offer of about 75k based on an online estimate.