The ME-U (or an individual ME device) can connect to the GX4816 via the built in ME port. It’s the second port on the box which will still leave the first port available for additional DX boxes to be connected if needed.
You will need to connect the Avantis with the SQ console. Since you are likely already using the built in SLink port to connect to the GX4816, you will need a GigaAce I/O card to put into one of the two I/O card slots. You will need to connect the SQ (via it’s built in SLink port) to this GigaAce card.
Long story short, a single Dlive/Avantis GigaAce card is really all that you will need to make all of this work IMHO.