In regards to IOS, any ISR router will work for you. The CUBE is a border element that will proxy and hide your internal network from external SIP connections, such as your PSTN provider not having direct access to CUCM or CME. CUBE is a little overkill for what you need.
The size of the router will depend on the number of SIP connections coming to/from your endpoints such as avaya and OCS. Its really just SIP to SIP and dial peers on the router to make this work.
The trick is will OCS *and* avaya talk the same SIP to/from the router. 2900/3900 or 2800/3800 will work fine for your application. If its small, you can probably get away with 1800 series
