Ideally your purchase price would be enough to let the owner pay off the lien that's foreclosing and all others. 

When purchasing our new family home we asked our lender if we could still purchase the home if we held onto our previous house, the numbers worked to keep it (cash flowing below the number I would ideally invest in, but it still cashflows positive).

A good way to compare might be to look up an amortization schedule spreadsheet (ideally an excel one).

@Kevin Heiden, ideally, your own income would be sufficient by itself to qualify for an FHA-approved loan for a 4-plex (ie. who wouldn't want that amount of income?)

Well mostly, the motivated sellers I deal with are generally not only willing to sell for a below market price, they are basically willing to give it away.

Do you know what price points, neighborhoods, and rehab budgets would be ideal for you?

I always recommend new investors start by defining their goals, then identifying the best strategy to reach them (given current macro conditions) and then what they are looking for in their ideal market. 

Movies a d chocolate both thrived during the Great Depression.

Your ads need to resonate with your ideal audience.Also if you are  not using the right budget, you may never get a lead, which brings me to the following step.You need to calculate your Cost/Deal.You also calculate your cost/lead.

What LTV % would you say your portfolio is currently at and where would you ideally like it to be on average most of the time?