The headset and stem won't work on the Nashbar (1" threaded vs. 1.125" threadless). Are the wheels in like new or good condition, or have they seen better days? The Shimano components may be good, but how low does the gearing go?
I think that you'll have better success in keeping within your $600 budget by looking for a good used bike (garage sales, eBay). There's good stuff out there, you just need to spend some time to look for it.