Theoretically, you should be able to produce a "radio ready" (RR) mix with nothing but the stock plugs in the Sonar line.
There is more to mixing than throwing a few plugins into an FX bin and having a RR mix. Mixing takes skills that take time to develop. It don't come over night and certainly not by throwing plugs in a bin.
I do use a lot of the Cakewalk plugs on my music because I think they sound good. Reverbs and EQ especially.
I, like many others here, have also bought some of the higher priced tools because they tend to make the job a bit easier. But, you still need to understand what they are doing and why, and most importantly, where, when and why you need them and also when you don't need them.
Feel free to listen to some of the music on my site. Most of the plugs in tracks and busses are Cakewalk plugs.
What I suggest is start recording and mixing and post the material in the songs forum and ask for feedback on the mix. There are a bunch of folks here with good ears, who will tell you what they hear and what you need to do to correct any issues.