I don't have personal experience with Olympic. The general rule though seems to be to avoid them. If I remember right, I think there were some threads about that carbine in question and the response was not positive.
Not all stocks are the same either. Some are better quality than others. I have a 4 pos. fiberlite(sp?) on 1 carbine and a factory Bushmaster 6 pos. on another. The 4 pos. is an el cheapo and it definitley shows it. Then you got the magpuls, vltor, ace and so. They are expensive, but they are very well made and usually pretty comfortable.
I have used a Rock River Arms lower on the one build I've done. Excellent quality, but they are a little tight. My other two are factory Bushmasters, which their lowers are pretty pricey. Avoid Hesse/Vulcan for sure, they will say the military uses them. They don't and they're junk.
You could probably find an assembled lower receiver with the stock on it for fairly reasonable and then save a little more and get an upper receiver at a later time.
Another option is use the $600 to put one on layaway. A lot of gun shops have layaway, and 14.5" Bushmasters are going for about 850-900 last time I checked anywhere near me. 20" are in the 800-850 when I bought mine.