The expert you are looking for is called a Land Surveyor, they're going to want 5k-15k, if you want to make any money on your project you need to build a house so you also need an architect, they will want 10k-50k, if you're in a hillside zone you will also need a geologist, they're going to want 10k, you will also need a biologist, they're going to want 2k-5k.

Since you're open to markets beyond NYC, you might want to connect with local investor meetups or REI groups in those areas to build relationships with agents, wholesalers, and lenders who understand the nuances of each market.If you're looking to break out of analysis paralysis, maybe start by narrowing down one or two target markets and running numbers on actual deals.

It can be done but it may take years to build up cash flow especially if you live in high cost area like CA.   

Apparently, not only did they sell at a loss, I didn't get any money back at all and they claim I owe them an additional $40,000 for "renovations" despite no work being done to the building at all. 

Are they currently investing in deals and building in this market.3.

We are building a place with HELOCs then after it is built we will finance it and pay off our HELOC, at least thats our plan. 

Try it out first test out whether or not you two are a good fit before building a massive portfolio together.Create an operating agreement for your LLC, and create a partnership agreement, defining roles and expectations.

The ultimate guide to building rapport and great relationships.

Bonus Tip: If this quadruplex is out of state, I’d highly recommend building a reliable local team (cleaners, handymen, etc.) and maybe consider hiring a co-host or VA to help with day-to-day tasks, especially if you scale up. 

However, just get in, see where you fit, and build over a decent pace where down the line you will be doing great.