Is that $54k an actual cashflow figure, net of all expenses including maintenance and mortgage payments? Or is it net operating income? Or is it ignoring long-term maintenance reserves?
I'd start by assuming a desired cap rate for the whole portfolio, and use that to arrive at purchase price. So if I wanted a 5% cap rate for properties that net $54k per month == $648k per year, that would mean I should pay no more than $12.96 million. If you think about it, this is also what the seller should be looking for: enough money to at least buy an annuity that would replace their real estate profit.
This is obviously a ballpark, within 20-30%. Next you'd need to determine what rate you'd get on financing, because that will eat into your profit and may force you to seek a higher cap rate.
I have no idea how bank financing would work on this, but I do know that a lot of investment property mortgages these days are requiring 25% down. They also don't want to see evidence that any part of the down payment is a loan. Talk to a mortgage broker.
At some point you should look for individual properties in the portfolio you don't like, and make sure there's enough money to be made on the deal that you could sell off the ones you don't like for a small loss and still come ahead generally.
I think you need to remove the urgency from this situation. That is a lot of property to buy (12+ units in one shot?) in a hurry.
I agree here, unless it's because the seller is desperate and that means you have a deal on your hands. The high risk of assembling a deal this quickly had better be paired with enormous reward for pulling it off.