I'd say that it's definitely worth to get an SSD - at least one!
If you can't afford a high capacity one (I don't know what capacity would be sufficient for you to go all the way with the SSD), then you should get one in the 256MB-512MB range and use it for the Hauptwerk cache only.
You can either set up Hauptwerk to use the drive for caching (I assume you are using Windows) or map the drive to be mounted in the cache directory.
I would not mess with external drives but if it's a good one, I don't see why you won't get a speed improvement.
My thinking against the external drive is that you can unplug it and then Hauptwerk will become non-working. I'd say go with an internal one and you'll have no worries.
Another setup with the internal drive, that could be even more advantageous, is to reinstall Windows and Hauptwerk to the SSD (and keep the cache location there also). You would use a classical hard drive for the sample set installation files and other large file storage. This way you speed up the loading of the OS and Hauptwerk and the cached sample-sets, but you need to be willing to re-install you OS. The first setup seems easier to do to me.