Mesh, I have done tons of projects with just the tools available in Sonar. That is, after Concrete Limiter became available. Mainly, as an experiment to see if any clients could tell the difference between using Sonar plugs or the Super-Duper stupidly expensive mastering plugs. NO ONE has mentioned a thing about how much better the results were!!!!
That is not to say that one way is better than another but I have found that MOST people can not tell the difference in a blindfold test. I have played master mixes to the clients of both versions and a lot of them preferred the Cake mastering version over the Super-Duper version.
Mastering has to do with the knowledge of how to use the tools that you have available to you. When you get right down to it, if you have a working knowledge of EQs, compressing, and limiting, you are more than 80% there. Just try your hand with what you have. Oh, and don't forget to do your research!!! The internet is full of info on how to master. Don't forget YouTube videos as well.