You should start off by looking at the list of certified Gateway devices. Look at only the items with a Configuration Type of Basic Gateway or Basic Hybrid; ignore the IP-PBX items:
http://technet.microsoft.com/en-us/office/ocs/bb735838.aspx#directThe most common devices used are typically the Dialogic DMG2000 and the Quintum Tenor DX. Both offer detailed configuration documents in their websites for OCS integration.
And it's pretty simple to setup a test environment in that you can connect the gateway to your PBX via a standard T1/E1 cable (depending on where you are in the world) and then after configuring Enterprise Voice in OCS just enable a few user accounts. If you have additional numbers in your PBX's numbering plan (preferably a contiguous block) then you can configure the PBX to route those numbers over to the gateway and then just give additional numbers to the few OCS users and not impact their current desk phone extensions.
Jeff Schertz, PointBridge | MVP | MCITP: Enterprise Messaging | MCTS: OCS