Worked a treat! Loved bringing this bad boy back to life and finding 10 year old playlists! Thanks very much.
Per a couple of the tips in the comments, I found it easier to start prying open the case from one of the sides rather than the top. When disconnecting the headphone connection, you’re pulling out a little plug (the descriptions and comments are a little confusing about exactly which bit you’re separating). Also, I didn’t find it necessary to remove the hard drive (Step 8) - You can just lift it up when taking the battery out, without disconnecting it (one less step to risk).