Buying Plug-in's for the Limited Edition

Yes, you can buy many of the Cakewalk soft synths seperately.
There are also tons of free synths and effects out on the web.

It is limited in how many simultaneous soft synths and effects you can use.
It can use fewer audio and MIDI tracks.
It doesn't have some of the effects and synths and processes of the higher-priced versions of SONAR, but you can get many of them and they'll work.
Any VST (or DX on 32-bit OSs) effect or soft synth should work with it.

I found a good way to add new plugins to Sonar 8.5 LE was to purchase X1 Studio when on sale,
and install only the plugins + sample content.  This adds new synths and VST plugins
to Sonar 8.5 LE, such as:
- Studio Instruments (4 instruments - drums, elec piano, bass, strings)
- Session Drummer 2
- Sonitus (11 effects including compressor with side chaining)
   (side chaining allows lowering any specified backing track when drums or vocal are playing)
- Boost11 (livens up mixdowns and vocals)
- TS-64 Transient Shaper
- TL-64 Tube Leveller
- Rapture LE Synth (200 patches)
- RGC Synths (SFZ+ Pentagon, PSYN II, Square I)
- RXP Rex player for drum loops
- Roland GrooveSynth (apparently it is for Project 5, which I don't use)
- Cakewalk Sound Centre synth
- Dropzone wave sampler
- TTS-1 Synth (256 patches)
- new version of Dimension LE (VST only)
This adds a ton of new plugins to Sonar 8.5 LE and defers / bypasses any need to
 learn / install / update / deal with bugs / convert projects to Sonar X1
 and gets free updates + a discount for future releases.
 
Later, when you have time, inclination, and perhaps a new Windows 7 64-bit PC,
you can install X1 Studio (32 or 64 bit) + Studio X1C Patch, and try out Roland V-Vocal,
Arpeggiator, Matrix View to trigger loops, loop library, step sequencer 2.0, etc.