POTN is http://photography-on-the.net/forum/
, a website/forum dedicated to all things Canon and frequented by many pro photographers. If you search you may very well find answers to your questions, but if you don't and post a question you'll likely get prompt and authoritative replies. You'll have to join first, but it's free.
"Nifty fifty" is the basic f1.8 50mm lens, EF of course in this case.
What is a good price of course depends on what it is you're buying. New price for the MkII 16-35 is approaching $1400, so I regard a very good price as say $1100 for new or maybe $650 for used, depending on condition of course. I'm not really being serious about that lens as although I'd really like one, and in fact will get one at some point, it's a purchase I'd have to make for myself. If you saw some cheap extension tubes then that's different!
A 50mm gives the traditional standard field of view on a FF digital camera, or a 35mm film camera. To get the same field of view on a Canon cropped sensor camera like the 40D you need an 80mm.
I can't find the POTN thread on the Tokina 12-24 that I was looking for, but here http://photography-on-the.net/forum/showthread.php?t=631427&highlight=Tokina+12-24
is one for sale.