Download a wow addon manager like wowmatrix. Google Wowmatrix
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Inside your World of Warcraft game folder you will find a folder called "Interface". Inside is a folder called "Addons".
Addons provided on abovementioned websites usually come zipped, with the files in a folder with the addon's name.
Simply copy this folder entirely into the "Addons" folder. Make sure to quit WoW and restart it. At your character selection screen, at the bottom right, is a button "Addons".
Click it to see a list of addons you are using.
Make sure all addons you want to use have a tick in the box, and that the box "Load out of Date Addons" is ticked, in case you installed an older addon.
Log into your character, and the addon should now be active.
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Yes, you can use any addons that a full account can use. There is no limit.
There are two major sites that deal with World of Warcraft addons. Curse and WoWInterface (See related links for the addresses). Google code is starting to have more and more addons as well, however, those are a little harder to find.
Considering WoTLK is 18 GB Normal, i'd keep atleast 25 - 30 gb for catacylsm inclusive of addons
no
When you install WoW, it install all expansions and patches up to the current client. This is even true if you do not upgrade your account to BC or Wrath. So it will take about 15 gigabytes.