Could I install my OSX system into and boot from the external hard drive
Yes, this is possible. For full details refer to How to set up and use an external Mac startup disk. If you have any problems, feel free to come back here and search for an answer (or ask another question if you can't find a solution).

In terms of your Samsung MZ-750250B, if you actually meant a Samsung MZ-750250GB then this is actually a SSD and supports a sequential read speed of up to 540 MB/s and a sequential write speed of up to 520 MB/s. I think you'll find this is more than enough for everyday use.