Just for the record, I agree on the CA2A being excellent and worth the money, and the Concrete Limiter is worth it especially if you don't have other good plugs yet that could replace it.
I'm not a huge fan of the other PC compressors/gates myself but that has to do in part with the PC format (which many people really really like). You already have the SSL bus version I think.
I would say that probably the 1176, LA2A and SSL4000 bus comp are the three most famous classic compressors. It would be a great base to start from. So if you can only get one right now, perhaps the CA2A makes more sense than another algorithmic reverb, even though it's a great one.
Sorry to add to the confusion! :)
/goes off to buy more plugs