Okay, there are some important things you should know about SD card usage. The short of it is, when google released kitkat, it basically turned that nice SD chip you have into just a piece of plastic.
The long of it is more complicated.
Your SD card CAN be used to STORE data -- but you'll find almost no application (other than a precious few that are really kind of crap) that will WRITE to the card. Docs won't work. QuickPic won't work and neither will your music player (although google music MIGHT work, I haven't tried it.)
If you want your games to actually use the SD card, then you'll probably have to install the games ON the card -- otherwise they WILL NOT BE ABLE TO WRITE TO IT and your game won't work.
Whatever directory you store your information in on the external SD card is totally up to you. Call it whatever you want -- it doesn't matter.
You want to be able to store your files on the SD card -- most of us do. You can thank Google for the fact that your options of getting it TO the SD card are very limited. You can use My Files, which DOES work (although it's pretty crappy) and you can use a PC and ( I assume) a Mac direct connection to write to the card. As far as I know, no other method will reliably work. You'll hear people tell you that ES File Explorer (an excellent app) will work. It does for some, it doesn't for most.
The camera app on the phone will write to the SD card, so you can keep your photos there. However, if you do, you'll only have two very inferior apps to edit those pictures -- even the names -- and those are the apps that came with the phone. You can use something like QuickPic to look at the pictures, but all these apps (thanks again, to KitKat) don't work well in that you'll get EVERY SINGLE PICTURE ON YOUR PHONE, and you won't be able to hide any of them. What that means is that EVERY SINGLE COVER you have for any mp3 you have on the phone, will turn up in your search and your gallery will look like crap.
Again, the long and short of it is that if a 3rd party app needs to write to the SD card, it can't -- although many of these apps, if they're stored on the card, can write to their own directory. If you go into the Applications section of System Settings, you'll see that there are apps that you can move to the SD card. I haven't really done that with any of mine, but I suppose I'll have to at some point.
Good apps to get? Guess that depends on the stuff you like to do with your phone. I love Nova Launcher Prime and immediately put it on. 1Weather is pretty awesome too. CPUTemp is probably a good thing to have. Dropbox is now almost useless since it can't write to the SD card, but I still have it. EverNote, OneNote are good apps. So is Dolphin Browser.
Good luck with the new phone. It's a nice machine, but until we can once again root them, it's about as useful as an iPhone.