All the responses above answer you question completely. Everything required to make great recordings is included with Sonar except talent. You have to bring that. 
 
All that being said, chances are you are like most other Sonar/DAW users out there which means the following will most likely apply to you. You don't NEED any plugins not already included with Sonar. That being said, you WILL end up adding more. You won't need them, but you will buy them and you will appreciate them once you have them. 
 
The most important thing to remember that every bit of marketing every music software and equipment vendor puts out there is designed from the ground up to convince you that your output is suffering from a lack of quality that can only be improved by use of their product. You had no idea you needed the Led Brick Limiter Plus until you saw the ads. And now when you listen to all your recordings, they are all clearly lacking that magic sonic glue that only the Led Brick Limiter Plus can bring. And if you act now, you can get Led Brick Limiter Plus along with The Phil Spector Hair Compressor and the Abby Road Invoicer for the bargain price of $299 yada yada yada. Don't get sucked in. 
 
Only add plugs when they bring real benefit. That benefit can be sonic or it can just improve workflow. The point is don't get sucked in to the hype. If you add a plug, make sure it earns its keep and does something useful.

JohanSebatianGremlin
All the responses above answer you question completely. Everything required to make great recordings is included with Sonar except talent. You have to bring that. 
 
All that being said, chances are you are like most other Sonar/DAW users out there which means the following will most likely apply to you. You don't NEED any plugins not already included with Sonar. That being said, you WILL end up adding more. You won't need them, but you will buy them and you will appreciate them once you have them. 
 
The most important thing to remember that every bit of marketing every music software and equipment vendor puts out there is designed from the ground up to convince you that your output is suffering from a lack of quality that can only be improved by use of their product. You had no idea you needed the Led Brick Limiter Plus until you saw the ads. And now when you listen to all your recordings, they are all clearly lacking that magic sonic glue that only the Led Brick Limiter Plus can bring. And if you act now, you can get Led Brick Limiter Plus along with The Phil Spector Hair Compressor and the Abby Road Invoicer for the bargain price of $299 yada yada yada. Don't get sucked in. 
 
Only add plugs when they bring real benefit. That benefit can be sonic or it can just improve workflow. The point is don't get sucked in to the hype. If you add a plug, make sure it earns its keep and does something useful.

Man this is timely.  I am at my heart a minimalist and I just watched a documentary on minimalism that spoke in detail about marketing and the way people are wired. Marketing is strong motivator, no doubt.  But I don't like buying stuff for the sake of buying stuff anyway.  We all try to catch lightning in a bottle, but I'd really rather learn methods and techniques that would allow me to do a decent job with any gear I have at hand than to wander down a path of buying, failing rinse and repeat.

Tell me...does SPLAT come with everything I need to get a handle on audio engineering?

No, you have to add unbending intent, unquenchable desire and a lot of blood, sweat and tears. LOL.
JohanSebatianGremlin is 100% spot on. Learn to use the Cake plugs inside out.

Concentrate on methods and techiques as you said.
You will know when you need something more.
If gear was the determining factor, we would all have a shelf full of Grammies and a pocket full of change. 

microappIf gear was the determining factor, we would all have a shelf full of Grammies and a pocket full of change. 

I am stealing this for my sig line. Thanks!

As far as a SPlat minimalist approach goes,  I got on board with that idea last year pretty much by accident when I decided to pick up a secondary lap top during a B F sale ...
This machine did not have various SONAR upgrades on it , nor did it have a lot of 3 rd party plugs and synths  installed because I decided to keep it totally lean and mean...( my main computer had all that stuff on it anyway )
A very interesting thing started happening to me .
Since I didn't have a bunch of clutter on the secondary machine  I didn't have to wade through what seemed like thousands of other distracting things and choices just to get something happening ...
From my point of view it seemed like much less of an energy drain was taking place....
At the very least it gave me the opportunity to get more familiar with some of the new features and tools that come w SONAR ...
Now I won't say that I was magically transformed into some sort of a power user or anything like that ...
For me to make any sort of progress I have to treat SONAR as if it's a musical instrument where I'm sitting down consciously sitting there in the woodshed learning SONAR as if I was learning how to play a new tune that has a lot of changes to blow over ...
I did get a little better in a couple of areas and I still have a long ways to go ..
 
all the best,
 
 
Kenny 

yes =-)

No. 
 
All the plugins in the world won't make a poorly recorded track sound good.  Start with the basics.  Learn to use your mics to make recordings that sound good without any plugins, then you can learn to use plugins or external processors in a creative way, not as a way to fix a poor recording.

JohanSebatianGremlin

microappIf gear was the determining factor, we would all have a shelf full of Grammies and a pocket full of change. 

I am stealing this for my sig line. Thanks!

No problem, I'll make more.
I do appreciate the credit in the sig.

aspenleaf
No. 
 
All the plugins in the world won't make a poorly recorded track sound good.  Start with the basics.  Learn to use your mics to make recordings that sound good without any plugins, then you can learn to use plugins or external processors in a creative way, not as a way to fix a poor recording.

What else should Sonar include to make it so that someone could get a handle on audio engineering?
 
Sonar has all the tools and it makes no promise that what you record into it or produce out of it will be great.  Like all tools, knowledge and experience are needed to make something of quality using them.
 
And while getting it right at the source is important, it is only one step is a long chain.  It starts with the song.  If the song sucks, a great performance isn't going to make it great.  And if the performance sucks, the best mic's and recording techniques are not going to make it great.  And if the recording sucks, no amount of plug ins, barking dogs or car horns are going to make it great.  And if the mix sucks, no amount of mastering is going to make it great.  And if the finished, mastered track sucks, no amount of marketing is going to make it great (but it might make it a lot of money.)

The OP wanted to know if Sonar Platinum comes with everything needed to get a handle on audio engineering.  No, it doesn't.