You are correct about the MGI license. You can have a MAX of 6 of those on your system.

If you need more you will need to install an OAS card which gives you 16 MGI channels (you can have both OAS and MGI licenses)

The SIP license you are looking for is called a Non-samsung SIP extension (or 3rd party sip) license.

You are correct if you goto SIP rather then ISDN you will need MGI (or OAS card(s)) and SIP Trunk licenses.

It is a bit expensive to go from pure TDM (digital/analogue handsets and trunks) to a mix of TDM/VoIP trunks/extensions.

The OAS card will take up a slot in your system, you have 5 slots in total that will have your current trunk and ext cards in them.