newbie questions on multifamily apartment investing

[Jerry Jones]

I am a buy and hold investor. In the past couple of years, I have bought some SFR and 4plexes in phoenix with 30 years fixed rate residential loan. With the leverage, I am able to get ~15% cash-on-cash return. Now I want to get into the larger scale multifamily apartments. I have a few fundamental questions:
1. Financing and risk
After talking to a loan broker and us bank, it look like I can only get 5 or 7 year fixed rate commercial loan for multifamily apartments. While the rate is super low now, it's going to be reset after 5 years, to a higher (more likely higher) or lower rate. This uncertainty definitely presents huge risk with a big loan amount. It's possible that the future mortgage rate will eat up big portion of the return. How to reduce such risk?
2. Equity build up
With a 5 or 7 year ARM, most of my mortgage payment will go to interest with little equity build up. Then after 5 years, I repeat this process with a new loan again. Intuitively, it will take much longer to pay off the loan compared to a 30 years fixed conventional loan. So is buy and hold a good strategy for multifamily apartment investing? If not, what's the best strategy.
3. price/unit and ROI
I looked into MFA in the phoenix market. The price/unit is much higher for buildings with more than 4 units (>$40K/unit),compared to 4plex buildings(~$30K/unit). I just couldn't figure out why is that. We all know that costco has a better unit price that target! And I would expect a lower wholesale price for bigger buildings! Is this the case for other markets? With this high price per unit and generally higher interest rate of commercial loan, I would expect the apartments will offer a lower ROI compared to 4plexes. So what's the common ROI for multifamily apts those days?

Thanks in advantage for all your inputs!