Is there a reason why you want to use a 5300? (i.e. you have one lying around)
a 5300 can carry calls but it doesn’t have any of the features people expect from the PBX, such as call holding, call waiting, etc, etc…
A 5300 is just a gateway. If you want PBX features you need Cisco Callmanager at least one gateway. Callmanager can scale from a few users to 100,000s of users with Callmanager clusters.
This means it can be “right sized” for what you need.
Here’s a list of all of Cisco’s Voice software, Callmager is one of them and it interoperates with any H323 gateway.
http://www.cisco.com/en/US/products/sw/voicesw/index.html