Mounting an SD as internal storage will cause /storage/emulated/0 to be on the SD card. All Apps will be non the wiser, everything will work as before.
The SD Card will be encrypted (afaik) and only work in that device.
You should never remove an SD card which is formatted as internal storage.
You should buy a fast SD card. I used a 128GB Samsung card in my Moto X, which was SDXC class 10, and should have 20MB/s write and 80MB/s read. But is was so slow that I had forced reboots all the time.