At this rate, and achieving a 7% return, they will accumulate enough wealth to "retire" in just over 50 years.

Once you accumulate a certain amount of cash/credit, isn't it faster to accumulate by buying large apartments?

If you go this route, you should accumulate a list of vendors (plumber, pest control, appliance repair, maintenance man) that you trust and can call out for repairs. 

I would seriously listen to the above advice .On the other hand, you might want to calculate a rough estimate of what it's costing you to wait for a year (or however long it's going to take) to accumulate the $ down.

HARD MONEY LENDERS COME IN ALL FLAVORS,IM A BIG PROPONET ON MAXIMUM LEVERAGE,THEIR ARE HARD MONEY LENDERS THAT WILL LEND 100% FINANCING UP TO 65% OF THE ARV,ANOTHER WAY YOU MAY BE ABLE TO GAIN MAXIMUM LEVERAGE IS TO HAVE THE SELLER HOLD A 10% AND YOU PUT DOWN 10%,THIS ALLOWS YOU TO REDUCE YOUR OUT OF POCKET COSTS,USING FINANCING YOUR WAY TO WEATH STRATERGIES ALLOW YOU TO MOVE VERY QUICKLY IN ACCUMULATING YOUR REAL ESTATE FORTUNE  

So I would suggest to keep it simple: open Roth IRA at Vanguard, contribute max each year, invest in index mutual fund (such as Vanguard Total Stock Market) and when you accumulate enough to do a real estate deal then convert to self-directed IRA. 

To illustrate this point I put together a simple model comparing five-year investable cash accumulation for a high cash flow market vs. a high appreciation market.

You build capital by spending less than you make and accumulating the excess.

I am also trying to accumulate companies whose products are 'needs' not 'want' products.

Regarding paying them off, I will have about $40k accumulated by the time I graduate, so I don't see the benefit in dumping $40k into my student loans to pay them off when my monthly payment on them for a 20 year "mortgage" would only be about $250 at 4-5% interest rate.