Default Locked used phones and activation

Hi,

I'm about to buy a locked and used Note 3 and I'm planning to unlock it with a Sim unlock code from a website. The thing is, I don't know which models can be unlock through this method. Is there any limitation? For example, can I unlock a Sprint device to use on GSM carriers? Or T-Mobile/AT&T only?

Also, for carrier locked phones, do I need to activate it beforehand with the specific carrier Sim card? For example, do I need a T-Mobile Sim card, activate the T-mobile phone and then, use my unlock Sim code with my own Sim card? Or can I just skip the activation process and ?

Thank you so much for your time!