Gate flutter is when a solid gate carabiner hits against the rock face and the gate pops open potentially allowing the rope to slip out. You can show this by taking a solid gate biner and hitting it against the palm of your hand. You should be able to get it to pop open doing that. Wiregate biners don't do this because the lower weight/mass of the gate makes it so you can't really pop it open. It isn't common but it is something I'd recommend using wire gates for to prevent. Wiregates are also lighter so it won't hurt to go wiregate.

Slings are usually 12cm long and that is usually just fine. It's a good idea to have some longer draws for those uncommon situations where you need them.

You don't necessarily need to have a variety of draws unless you want to. I personally have 10 of the Firewires and 2 longer draws and that works out fine for me

So if I were you I'd go with the Quickwire or the Freewire. Both have wiregates on the rope end of the draw. The Freewire is a little lighter since both biners are wiregates if weight is something you pay attention to though their weight difference isn't huge.

Not much, most sport routes are bolted pretty well to accomodate normal length draws.

I am truly sorry. Your question makes much more sense from a gift point of view, and I apologize for my previous answer. These are awesome draws that any climber would love to be given as a gift. Like I said in my previous answer, if you are actually trying to stock his entire trip, then you will need 10-15 (which is expensive), but speaking from a climber's point of view, every piece of gear that you are given is welcome. In that case, one would be appreciated hugely. Sorry I was rude.Nice response Angus it proves you are strong to admit your wrong.

Have you ever been climbing at all? Like ever? I don't mean to be rude, but your question is something you really desperately need to know the answer to before you venture out on your own. To answer it, yes. You need to buy a set. When leading, you clip a draw (one of these) to each and every bolt on the way up. Again, if you didn't already know that, you shouldn't be leading at all. How many is in a set depends on where you climb, how long the routes are, how closely bolted they are, whether you feel like skipping clips (which you shouldn't), and a number of other factors, but count on needing 10 at the very least. I have 16 and have still encountered routes that needed more. I sincerely hope this was some kind of sarcastic joke, because a question like that gives me serious concerns that you will be in the next issue of Accidents in North American Mountaineering. Make good choices.