I’ve made $600 profit so far!

He owns a used car sales business, so what does he profit each year?

Schedule C on your taxes is titled "profit & loss for business" so with your STR income now being labeled as a business, (because it is) lenders will want to see consistency and growth in order to be able to use any of the income to offset the STR's debt services.  

I am reconsidering if I should hold the flip under the C-Corp or perhaps create a NEW LLC or S-CORP, take out a SBA Loan, fund the house, and pay back the loan immediately and have access to the profits for a future flip or to re-invest. 

Your bank loan may require Replacemwnt Cost, and he has ACV.Lastly, if they have a larger portfolio of 100 units or more, you will not be able to get thier volume pricing.At the end of the day, hopefully you can get it done for $800,.  

I signed on with them for two years and will be splitting profits 50:50.

Do you prefer replacing a single larger roof or five roofs on five different homes; do you want to keep going from home to home to keep replacing water meters, or you'd rather fix one main unit that has all five units attached to it; do you want to  pay your attorney five times for five house closings or rather pay a fee once to close a five unit home.

The larger the credit line, the lower the utilization. 

Both 3 unit houses cash flowed right out the gates and will continue to profit greater as I improve the places and raise the rents.

Can carve out responsibility and control of profit loss and agreement for common expenses within existing LLC.