This is the way I think I'm going. 2TB SATA drive I think.
It's a mid 2010 27" iMac 3.2GHz i3. The 5k would be significant. However, I can't raise the £2200 I need, and a 2TB HD can be under £100 at the moment.
So that's the choice, although the HD swap isn't trivial I don't think it's beyond my means.
How to guide.However, until payday on the 25th, I think I'll flatten the drive tomorrow and restore from Time Machine. Of course, this is happening when I've got a video edit to finish...