When buying a performing note, the price I am willing to pay ultimately comes down to the yield I am targeting and the interest rate on the note. My target yield will vary based on the riskiness of the loan. Personally, I would never pay 100% of UPB for a loan. What happens if it stops performing and you now have foreclosure costs?
Paperstac is a good place to look for loans to buy. You can also network with private investors who trade with each other all the time.
Most states don't require you to have a license, but a handful do.
When buying a note the main thing is verifying the value. This is tricky because you can't see the inside of the property. That's where a lot of the risk lies. You should have a realtor on your team to help with this. Also pull an O&E report from a title company, and have an attorney who is licensed in that state review the chain of title and alert you to any liens or other issues. Although talk to them about the process, length of time, and cost to foreclose in that state if you need to.