I would get the Combined Operations pack if you don't already have A2, it adds both A2 and OA. IMO it's worth is because there are plenty of good missions for A2 as well and if you plan on joining a squad (definitely recommended, much better than pubs) they will probably play A2 missions sooner or later. I don't know the price of Combined Operations.
The first beta of ACE2 for OA is already out (4 days ago I guess) but it's currently only released through the SIX updater and Kelly's Heroes updater. A stable version of ACE2 for OA is not out yet.
ACE isn't really needed, except for missions that require it. Or, if you're in a squad ('clan' is a swear word in the BIS community) and if the server admin or team leader decides that he wants to use ACE during a non-ACE mission, the rest of the team can then also load ACE and it pretty much becomes an ACE mission then (if all of the players enable it).
If you buy it, be prepared to deal with bugs and minor annoyances, but the many great experiences make up for it.