The only guarantee when you buy a property is that you are about to spend a pile of money. 

In order to Legally Add/Legalize the Basement Unit, you would need to obtain a Building Permit and receive approval through the Department of Buildings (in addition to the Department of Zoning which you already to have the approval for). 

Looking to use it as a getaway from the city, maybe spend 2-3 weeks every 2 months there.

The structure often depends on how involved you are/want to be, and in addition, what % of the capital required you're providing.

It's okay to spend some time upfront shopping around for ideas but I would definitely recommend that you do not fill an app with more than a few, and DEFINITELY don't let them all pull your credit.

We would like to get a HELOC as down-payment for an additional residential property.

Additionally, we are on the lookout for reliable legal or real estate platforms that could serve as references or resources.If you have any personal experiences or recommendations in these areas, please feel free to share them.

Personally I spend more time keeping up with changing legislation, Fair Housing Laws etc than I do anything else.

I'm fortunate to have found a great GC to work beside me that will be our Project Manager to pull permits, and Subs/Handyman to complete all of the required work.We have some additional capital on hand that exceeded what we needed to complete our first two projects and recently came across a deal that we didn't want to pass up that our crew is able to take on.

Then if I end up spending less than what I was funded for, I would use the remaining money to pay off more principal.