HD video editing and external hard drives

Hello, I have a mid 2011 mac mini with the following specs:

2.3 GHz Intel Core i5 processor

8 GB 1333 MHz DDR3

Intel HD graphics 3000 512 MB

500gb SATA disc

Running Mavericks 10.9.2

I use it mainly for image editing software and it runs perfectly but I have recently started using it for HD video editing in FCPX and I found it sooo laggy. I know I could be using a greater computer but I can't afford a big upgrade just yet. Anyway, I've read that keeping all the media on an external drive would help the editing go way smoother, so here comes the question: which one would you recommend? Or will it even make a big difference?

I've taken into account this one (I can afford something around that price) but I'm not sure if the 5400rpm will kind of make the thunderbolt speed not as fast as it should be? I've been reading a lot about external drives but I'm still very new at it so I'm hesitant about what to do.

Thanks in advance.