picked up an '08 Chevy this weekend

[Low87]

Finally got tired of the '89 K5 as a DD. Found an '08 Chevy CCSB Z71. High miles but the carfax maintenance log was pristine (every 3 mos at the dealer, regular service, etc) and all the important stuff checked out.
The body has some "character" which I kind of like since I'll be using it for work as well as a DD. Nothing big just a few dings.
There's a few things I want to replace straight off;
-Pass. side mirror is cracked. Turn signal inside still works. Can I just replace the glass, or do I have to replace the entire mirror?
-Driver side taillight lens is cracked. Is there a good aftermarket, or is GM the way to go?

On the air intake tube there is a box attached to the front, about halfway. Any idea what that is?

I've never owned a NBS before, trying to get out of the squarebody frame of mind. Any good parts sites?