The website linked to below tells you which of the most popular coins have the highest market capitalization.
http://coinmarketcap.com/Some major alt coin trading exchanges are linked to below, but there are loads more.
https://bter.com/https://www.cryptsy.com/If you sign with them you can store small amounts of your alt coins in their online wallets. You can always install a coin's wallet on your own computer, but there is usually a limit to how many you are prepared to install.
I cannot advise you about the rest of your queries because I'm fairly new myself.