1/ Depends what it is, if it's a decent tourer with long range tanks you'll start to notice, if it's a bog-standard Cessna, you won't.
2/ Probably, yes.
3/ Standard usually works fine.
4/ Can't say it's ever impressed me greatly in a spamcan.
5/ Pass.
6/ Not really, what you really want is 4 the same so they're definitely compatible through the intercom - and you're very unlikely to buy 4 identical headsets second hand.
7/ Probably, but a pain if one goes U/S
Personally, I'd recommend going for the cheap (but really very good) headsets Harry Mendelson sell as own brand. Buy 4, and you'll probably get a discount, bag, and possibly a spare intercom box as well.
G