You can have a 5:1 margin account, like the one you describe, as an individual investor: If you have 100K to put in a brokerage account with your own money, you definitely have enough money for a brokerage to let you do it. It's just that it requires attention, because with a 5:1 ratio, a 5% loss just became a 25% loss. Ignore the market in the wrong week(say, in early October 2008), and all your margins get called, and you are in deep trouble.
Still, if you have 5 $100K rental properties, and you aren't bleeding a lot of money for a company to take care of it all for you, chances are you are still paying quite a bit of attention to your apartments anyway.
As far as whether mortgages or rentals are more expensive, it completely depends on the local market and local laws: In a pro-renter environment, rental is more expensive, because nobody wants to rent apartments out: People would rather let apartments sit empty. In a pro owner environment, at a place losing population, renting might be cheaper.
People generally don't buy houses in order to rent them. Usually houses are rented out when the owner is moving and decides to rent it out rather than sell, at which point the owner has a substantial amount of capital in the house.
Rental real estate works on leverage to a greater degree than is possible with stocks. Most banks won't make loans to invest in the stock market. They will make loans for investment property. So if you have $100,000 you can buy maybe 4 or 5 $100,000 rental properties, and have your tenants pay off the loans. Not as likely that you can take $100,000 and buy $400,000 - $500,000 of stocks, at least not as an unsophisticated individual investor.