Firstly, welcome to the forums!
If you've played 3rd Ed. you'll probably know most of the stuff you need for the setting: watch astral security, be prepared for grenades and the salsa they create, etc.
Specifically with respect to 4th edition, I've not had too much of a problem with twinked out characters, but that's probably more due to the people I play with rather than the edition itself; it's been proven here that, like Ravor said above, it's relatively easy to create monster dice pools at character gen.
I've also noticed that in my 3rd Ed. games, about 6 or 7 players was relatively easy to game with. But 4th Ed. seems to provide more generalist characters with more skills at lower ratings. As a result, 5 has been the optimal number of players for me so far.
Watch out for the matrix. As you can tell from a variety of threads here on the DSF, the matrix rules are open to interpretation. Remember that as the GM, if something doesn't make sense to you, don't allow it. If you don't think cameras should have a signal of 4 and be hackable from the parking lot down the street, then give them a signal of zero and they're only hackable if you stand underneath them outside of any cover.
For magic stuff, the biggest oops I've seen in game is people with Adept characters who don't pay attention to their maximums with respect to powers like Increased Attribute and Improved Ability. These powers do have caps but some people don't take that into account during character gen.
Personally, I wouldn't worry too much about the rules for reputation (i.e. street cred, notoriety, and public awareness) until you've got the rest under your belt. I've only recently begun to incorporate those concepts into my games after two years of playing 4th edition, not because the rules suck, but because they're icing on the cake.
And, finally, I've found that the grunts, etc. in the SR4 rules are a little too light for full-fledged runners. Beef their stats, skills, and equipment up a little bit (depending on your characters) if you want to create a better challenge for people.
And, no -- there are no prices/availability for street drugs.