Kind of a loaded question... the SX-200 is still a thing with Mitel, but it is only supported for Hospitality installations. Mitel has tried to kill this product for years, but it's popularity in the hospitality market has kept it alive. The MX controller has been retired some time ago, but there is an available CX controller that would likely work, but it only supports ~100 phones maximum. The AX platform is designed for up to about 50 IP phones and a couple hundred analog stations (for hospitality installations).
With the current release of software, you can add Mitel 53xx series phones (except the 5360) via IP. They must run on a VPN or direct LAN routed connection though, the controller does not support NAT translation (ie. no port forwarding).
Also, all phones must be licensed individually... You will need to engage your Mitel vendor to do this.
No modern platform still supports the digital peripheral bays, and digital phones on an SX platform are considered obsolete.
Honestly, if you need to expand, I would consider beginning a migration to a MiVB or hosted platform.