I also think you should get it from your LBS. You need to try and haggle with them, though. Maybe they can come off that price like 50 bucks or something. OR, maybe they can also throw in some pedals/clips/straps.

The thing is that you may not anticipate anything happening, but it still could. Specifically, your lockring and cog needs to be nice and tight. Also, installing the stem can be tricky. You don't want your handlebars sliding downwards while you ride, but you also don't want to strip your bolts or rip your star nut out of the steering tube. Also, the chainring bolts need to be tight and you will need a special tool for that. And it wouldn't hurt to make sure your wheels are trued (12 bucks each). As you can see, there are many things that need to be checked.

If you are mechanically inclined, i would suggest that you just spend your extra money on some tools and learn to do this stuff yourself. You will need pedro's tool as well as the park tool 3 sided allen wrench. Those two tools should be good to get you started, but you will still need to have a LBS help w/ cog/lockring. My LBS charged me 4 bucks to install my cog/lockring and tighten it.

The choice is yours. See if your shop will come off the price at all, and if they will then buy it from them. If you think you can handle all of these things, then buy the tools and do it yourself. Learning all of this stuff and buying the tools will be cheaper in the long run.

Also, GREASE EVERYTHING. Any place where metal touches metal. And definitely grease the hell out of your cog/lockring.