I think you can add 2 expansion carriers to the Magix for a grand total of 3 carriers.
The Merlin Magix evolved from the Merlin Legend. The R7 Legend (final release) has the same 80 line/200 extension limits as the Magix. The Legend will support Classic Merlin telephones while the Magix supports 4400 digital telephones.
Take a look at the 4400 series single line telephone. It might be a suitable alternative to using S/L analog telephones. If you go with that telephone, you will need only (7) 024 TDL modules. Those modules feature Amphenol connectors which can help make for a neat installation.
Avaya discontinued the Magix back in 2006. Available hardware, for the most part, is now refurbished, but there is a lot of it out there and will be for a long time to come.
Also, if my memory serves me, it has been suggested that there are better choices for hotel/motel voicemail applications than the Avaya Merlin Messaging. The brand & model escapes me, but one of the lead posters might have that information.
If it ain't broke, I haven't fixed it yet.