For an "external booter", I'd recommend that you go USB3 instead of Thunderbolt. It's more economical and will give you boot/run speeds that will be virtually indistinguishable from Thunderbolt in day-to-day usage.
If you're considering an SSD as your boot drive, you don't need a 3.5" enclosure -- a 2.5" enclosure (or a USB3/SATA dock) will do fine.
You can get a USB3/SATA dock for under $30. Then, just drop an SSD into it, and you have your "booter". I'd suggest plugable.com as a possible source for the dock (no financial interest).
For an SSD, I don't think 32gb is going to be enough. You probably want _at least_ 128gb, I'd recommend a little more (I picked up a 180gb IBM 520 series drive for $130 a little while back). A good SSD of sufficient size is worth spending the $$$ on.
Once you have your external drive ready, initialize it with Disk Utility, then use CarbonCopyCloner to "clone" the contents of your internal drive to the external. You can use CCC free for 30 days from the time you download it. CCC can even create a recovery partition on the new external drive, if that's what you wish.
Once your external booter is set up and running, you might consider keeping a "spare boot partition" on the _internal_ drive, too. It's always a smart decision to keep a second, fully bootable source nearby. Someday, you WILL need it.