Forget the DVDs forever.
These are old, slow, and they will install an outdated version of the sim that will require to download several tens of gigabytes of updates anyways.
So better download the latest version from scratch, really
I bought the sim on Steam. Gave me some freedom to define the location of the community (addons) folder as well as the destination drive to install the sim.
Steam by itself downloaded only 400 Megabytes for the core program install.
When launching that, the program internally downloads the rest of the simulator (several Gigabytes).
Once the sim is installed, you might also have to go into the settings/profile to reach something called the "content manager", that will let you download additional things, like the "world updates" for Japan and USA, or the premium planes in case you didn't choose the standard version (cheapest one).
I'd recommend you to buy the standard version.
The planes in standard version are not encrypted, so they can benefit from better mods made by the community. The A320 mod by a team called FBW is probably the best example.
The planes in the premium/deluxe version are encrypted (to prevent they appear as free downloads anywhere....), so they cannot be easilly modded by anybody.
I'd also recommend you to buy through Steam. I was already using Steam for other games anyways.