My budget for recording software is only slightly higher than the price of Cakewalk, so I'm qualified to answer here.
CbB actually comes with a very capable drum kit; I don't know why nobody has mentioned it.
SI-Drum Kit has several good-sounding drum kits and many patterns.
And speaking of patterns, CbB also comes with a ton of MIDI groove clips, which may be accessed via the Browser pane on the right. They work well with SI-Drum Kit.
To fill out your synth collection, go to
www.pluginboutique.com and register for an account (be sure to sign up for their newsletter, this will entitle you to a free license for iZotope Ozone Elements 8), then get AIR's Hybrid 3 or Vacuum Pro, or both, which are on sale for $1 each.
Be sure to also download the free expansion packs that they have for whatever synth(s) you buy. If you're feeling rich, buy some of the $1 expansion packs.
They are both top-notch, amazing synths. If you only want one, probably just go with Hybrid 3, but why not get both, it's only a buck. They will keep you busy for a long time. EDM-oriented, but versatile enough to do other things.
KVR Audio is a good site for finding free plug-ins. I check it almost daily.
This forum has a Deals subforum that is worth checking, too.