Don't forget to try the Nomad Factory bundle that't included with Splat.  They may satisfy your quest for additional plugs for quite a while.  Also, Blue Cat makes some seriously good and free plugs that complement those in Sonar.

SPlat ProChannel is really all ya need to do amazing stuff...the current sale on the PC leveler is a must have.
 
I sport a thousand plugs... I use maybe 10 of them... total waste of money!  If I were you I'd invest in Omnisphere 2 and a lot of Plug in Guru O2 packs... will radically improve your creative chops and songwriting more than ANY product I know. BTW, just go to youtube and subscribe to the pluginguru vids.... you will learn a bucketful of awesomeness for FREE

yorolpal
 
Now third party INSTRUMENTS...is a whole nother thang!

^^^^ THIS ^^^^

I love this forum - just brilliant, in depth thoughtful comment. Thanks for taking the time. I'm not a novice, I started with an 8 Track back around 2001, and have been using DAWs since about '04, Sonar since '08 I think. During those early years I was forever on KVR and picking up FX, some of which I actually used. Then you change computer, forget what you had and realise you don't miss those things.
 
Upgrading recently to Platinum recently I had already pretty much come to the conclusion that it contained virtually everything I could possibly need. 
 
So, I am very fortunate in that before the end of 2016, I will have more time to spend on my passion for making music. And that will be devoted to learning to use what is under my nose, including an annual sub to Groove 3.
 
And I'll get brave and post a song or two for critique, been a few years now!

I agree with Dan and would ask, if there is some well produced piece of music on the net and you don't know anything about the production can you tell what compressors and EQs were used to mix it? What compressors and EQs were used to master it? If not, then how important is a particular tool? What is important is does the tool do what you want it to?
 
As mentioned above I like Nomad Factories' Blue Tube bundle that came with Sonar and use the pultec style EQs as well as the oilcan echo. I think both work well with guitar. There are quite a few tools available in platinum.
 
Having said that, I have upgraded some of the PC tools over the years. I use the PC concrete limiter and CA 2A leveling amp quite bit, as well as the PC-4k expander/gate & compressor occasionally. I also use the VKFX. The VKVX are pretty nice used with an electro harmonix key9.
 
And of course youtube is your friend as well as the cakewalk videos.