Just to add to the previous magazine question, you have a couple of choices if you want the mags with the window to see the rounds in them.
Magpul Pmags are the most popular and considered one of the best and most reliable.
Lancer magazines also have the complete see-thru mags.
Here is a google image of a Lancer L5 magazine next to a Pmag:
I've never tried the Lancer mags yet but they look pretty cool so I intend to buy some to try out. They offer the 20-round Lancer mags for $10each at Aimsurplus
hope that helps.