Don't forget to add your time to work out the data bit format and then programming up a micro processor to do the mute function etc.

For now I'd go with a manual switch and get that bit working
[at least its something you can use], then go from there.

Personally, if you feel comfortable with hacking the OnStar system and maybe blowing it up and your head unit etc - then go for it.
If you want a simple interface with a simple installation then the 3rd party add-on kit sounds ok.

Plus, I'm sure you could find a cheaper piece of kit if you looked around on the net a bit - but you'd still be paying at least $70 or so I think..

Its your decision.