1) Yes, it has the option to clone your HD. However when I used it back last August, I discovered that if you clone to an external drive, it will not make it bootable. So, if you want a bootable backup, you have 3 options:
a) Install the new drive internally before cloning, if possible.
b) Make a backup to a different partition, then restore. This is what I did last August.
c) Use a different program to clone. There've been several new versions of the Easeus program since August, so maybe that's changed, but that's what I discovered back then. There are hundreds of backup/restore and cloning programs out there and many are free, but the Easeus one is what I've been using for several years with no problem.
2) A bootable disc is one you put in your CD/DVD drive, then turn power on and select the CD/DVD drive to boot from.
3) The last hard drive enclosures I've used are made by Inland. I've used other brands, but I've come to trust Inland. I can't remember have any of their devices to fail on me. YMMV.