Figuring out local CAP rate

[Brian Barfoot]

Background: I have about a half dozen SFRs that have been successful for the last few years. I'm looking into picking up my first multi (I will give details about the deal in a separate post because I definitely have questions on how to put it together wisely) and it seems to me that the property in undervalued. It's a four-flat residential on the second and third floors with additional two business offices on the ground floor for 150K in a small city in upstate NY. The apartments haven't been rented in over a decade and need probably 20-30K to be fully rented but when they are they'd bring in roughly 3K/month. The plan is also to convert the offices into residences (another 10-15K) in a year or two which would bring another 1500/month or so. 

So the question: If this place brings in $4500/month all I need for the valuation is the cap rate. A previous post on valuation said:

"Find out the true NOI of the subject and then the prevailing market cap rate for similar properties." 

But I don't know how I'd find the local cap rate. I'm getting a commercial loan based on the purchase price but I'd assume I can refi the loan once the units are rented for a cap-based valuation. Can someone help me to figure out the local cap rate? Also, is it accurate that a commercial lender will reevaluate the valuation once we are fully rented if we want to take some equity? If this is a 10 cap it's a half million dollar property unless I'm missing something.