funny I was looking into the same subject today!
I have recently replaced the battery in my old and forgotten x5 iaudio and very pleased with the results.
Now I'm thinking about upgrading the HDD also to something newer. I've seen posts of people having success with CF cards, but a SSD would be even nicer.
The thing is I have a hard time figuring out what kind of connector my old HDD exactly uses it's a Hitachi MK2006GAL 20GB disk (same as the old ipods). As far I can tell it's 50-pins ZIF connector, could you confirm this? Or can someone give more specific details about these drives connectors? While searching for this type of drive it appears they are pretty rare these days (maybe I haven't looked good enough) and the ones You suggest are very pricy, over 400$ for 128 GB and + 200$ for the 64GB versions.
Though there are cheaper ssd's out there with different connectors, so using an adapter and possibly a smaller 1 inch SSD could be a cheaper solution.