Mastering Software, Sonar 7PE or Third Party?

You can do it all with the provided tools, save for one: SONAR 7 does not include a good mastering limiter. Your best bet is investing in Ozone. It will give you the fastest, easiest results if don't quite know what you're doing yet.

For around $80 you could purchase Wave Lab which comes with the tools and proper layout for mastering. Sonar is a great multi track recorder, Wave Lab is your mastering 2 track deck. 

I use Wavelab 7 for the metering and other coding, Montage, report tools it provides. For plug-ins, I use these frequently:
Flux Alchemist for Multi-Band Compression
Algorithmix Red or DMG Equillibrium for surgical EQ
Waves JJP PuigChild for general "glue" compression
Elysia Alpha for M+S compression needs and some "glue"
DMG Compassion for Transient-saving
Waves Kramer Tape for a bit of smashing to Tape
Waves L3-16 for limiting
 
Ozone is an excellent choice, however. I wouldn't sell them short. Fab Filter Pro-Q is also awesome.
 
Any of these may have a less costly alternative, such as using Ferox for the Tape Sim, but these are the plugs I routinely depend on.
 
My advice is: Try and try again and again with the tools you have. Grab some ultra-cheap or free plugs and try those. But MOST of all, know what to expect from each one. The more you know what you can do and how each one works, the better you can make cheap or free plug-ins work as well, if not sometimes better than using expensive plugs and not knowing how they really work. And not all expensive plug-ins are always the best for your project. Case in point, the Algorithmix Red costs over $1200, but does a great job. The DMG Equillibrium is about $250, and also does a fabulous job. Many times I'll find the DMG Audio EQ gets used vs. the Algo Red. That's because it's the right EQ for the project.