You’re not going to find agents that will typically offer to perform UW work for you on the size of deal you mentioned, but they will for larger properties.if you’re interested in learning underwriting, I would recommend downloading some pro forma excel models to get a better idea of how they work.

Have you run a preliminary pro forma to confirm the deal works financially?

Saying you need to form a company is a little premature, try the first few individually so you can get better financing.

Likely, their only profit will be any residual cash flow left over after they've paid retail for everything along the line (as you said).And will that cash flow conform to pro forma predictions?

I'm not an attorney;Have an attorney form a LLC that will hold the property and give a percent of ownership to each investor based on the amount the contribute to the total investment, or whatever you agree on.Option B; go with a loan from them that you pay a flat interest rate on, for example 10%, they can have 2nd position on the lean.

I will state that most pro forma I see under estimate the maintenance/cap ex. 

Good point on the compounding, even just frontloading heavier increases to the earlier years will inflate the pro forma.

Every time you buy another property form a new one and hold the new property in that one...so on so forth.

As a Real Estate Professional in the Tampa - Sarasota market area I can attest to the hundreds of pro-formas that I put out quarterly. 

Hello Taryn,Have you considered securing hard money for short-term use to complete the apartment scope of work and then refinance for a conventional loan either based on "As-Is" (pro forma market rents w/ no tenants in place) or "As-Stabilized" (fully rented) condition whichever the lender will allow?