I really wouldn't recommend going external.
I put an SSD into the iMac my mother uses in her office a couple years back and it has made it much quicker and able to cope with Mavericks and Yosemite. I still use it a fair bit as it's easy to get to where it's located in the house.
Her uses are pretty simple still, Mail, Safari, Office, iTunes, some iPhoto/Aperture and the like. But It'll still run iMovie just fine too. I didn't have my MBP on me and I needed to stitch together a quick movie off my camera (1080p) and it handled the task pretty well for it's age and limited hardware. I was pleasantly surprised even if it took a while to export (2.0 GHz Core 2 Duo).
I did it myself and it really wasn't that hard. You just have to read a good guide (iFixit) and have the tools (torx bits and a suction cup, and tape if you go my method

).
A simple 128GB SSD is dirt cheap these days ($55 on Amazon) and well worth it. Make sure you've got at least 4GB RAM too. It'll run just fine.
Yeah, you could put it towards a new mac, but for how cheap a decent SSD is, why not go for it? I've put a few in some '07-'08 MacBooks and the users are quite pleased. They don't care if they are stuck at Lion, it does what they need it to and is much cheaper than a new model with power they don't use.