How much of the $650k do you have to borrow them to complete the build?

Your desire for control and liquidity are likely the two biggest assessments you need to make first, as this will start to narrow down your choices very quickly.

Start with your primary, rent out any spare bedrooms "house hack"Use that to practice upgrades, kitchen, bathroom, curb appeal... etcNow you have experience as a landlord and flipper, use that to buy a distressed propertyAdd 40% equity to the property through home makeover, refinance into a low cost mortgageNow you are cashflowing a healthy margin on a second property, use that to build equity over time, cashout later and buy your duplex/quad.If it's going to take you 10 years to save up a down payment for a small apartment building, yeah it makes more sense to start growing your portfolio with cheap SFR flips instead.Every market is different, every borrower is different, every property is different, it would be ill-advised to recommend anything without recognizing the individual's nuanced goals and objectives with their next acquisition.

I also don't use them on my personal, but that personal choice. 

I think this is doable with my midwest market, but difficult with current borrowing rates.

What’s the biggest red flag you watch for when vetting borrowers?

The investors in the secondary market have a very good grip on the prepayment risk associated with a drop in mortgage rates and use pricing or yield to offset this, which is why you so often hear of borrowers paying points and lender fees.

RCN Capital - Never used them, but seem to be legit Roc Capital - Used them, be careful here, if you don't do your proper due diligence with the deal, you will end up with an unhappy borrower.

I have no shame driving for Lyft if it will give me a rental a year without using any borrowed money!

If I did my math correctly, on a 3,600 SF triplex where you buy the land for $150k, you generate a gross profit of about $84k (based on a $140/sf build cost and sale price of $205/sf), before you pay interest on any borrowed funds or closing/commission costs.