Along with Sonar I use a few VSTi plugins (notably Kontakt Player for Steven Slate Drums), a handful of VSTs I've collected over the years (as well as the plugins for my UAD card) and that is about it.
Sonar - and pretty much any other DAW - are virtually ready for demanding studio use straight out the box, and anything else you add is down to your own specific requirements or preferences.
Some recent big hit songs have been done entirely ITB with minimal gear no much more expensive or dissimilar than what some of us home-recordists work with... its widely known that 'The Fear' by Lilly Allen was produced more or less using only the stock Logic plugins (including the bundled basic amp sim), with only one or two pieces of analog gear employed on the front end on the way in.
So its perfectly feasible to use a DAW and just the stuff that comes bundled with it. In terms of recording you're not going to run into any limitations... the main place where you'd need to add stuff is if you want specific sample libraries, VSTi's, etc.
