Data first, since that's the only thing you genuinely have to buy. Notice-of-default records are public in every state, and in a lot of counties you can read them free on the recorder's or clerk's website, or in person. That costs time. Paid platforms exist because they pull those filings daily across many counties and attach skip-traced phone numbers and addresses, and they typically run in the range of tens to a couple hundred dollars a month depending on how many counties and records you want. Skip tracing is often billed per record, cents to a dollar or two each.
Then mail or calls. A few hundred addresses at postcard rates is a small number. Consistency costs more than the first batch, because most owners don't respond to one touch.
On licensing: buying property for your own account generally doesn't require a real estate license anywhere. What does draw licensing rules in a number of states is negotiating or arranging a loan modification or foreclosure rescue on someone's behalf, and several states also regulate foreclosure consultants specifically, with required contract language and cancellation rights. Whether your particular script and process crosses that line depends on your state's statute and on what you're actually offering, so run your script past an attorney licensed there before you scale it.
One cost you haven't counted: title work. These properties collect junior liens, tax arrears, and judgments, and you want a title search early rather than at closing.