A master lease means you lease the entire building at a fixed price and then rent the individual units out yourself, keeping whatever's left after your payment to her. Your profit is the spread. Right now the spread is negative $900 a month, and everything depends on whether you can close that gap.
So work the gap. Fill the vacant unit at, say, $1,250 and you're at $4,750 against $4,400, a $350 spread. Bring the three occupied units toward $1,250 as leases turn and you'd be at $5,000, so $600. That's the whole business case, and it rests on those market rents being real. Pull actual comparable rents for that submarket before you agree to anything, because her opinion of what the units should get is not evidence.
What the numbers above leave out is that the $900 shortfall is yours to fund from month one, plus turnover costs. If it takes four months to fill the vacancy and each turn costs $2,000 in paint and cleaning, you can be $8,000 to $10,000 down before the spread ever goes positive. Have that money set aside as cash, not as a plan.
Also get clear on who pays for the water heater. On a master lease the maintenance split is negotiated, and a building someone's tired of managing often has deferred work waiting. Ask for a dollar cap on your repair obligation.
Keep the option to buy at $520,000 in a separate document from the lease. If you fix the rents and the building appraises higher, that option is where much of your upside sits.