[mothered]

I’ve opened a claim with PayPal, uploaded all the ‘proof’ - done it as an ‘Item Significantly Not As Described’ due to missing items.

Given you're claiming the Items were missing, you should've opted for "INR" (Item Not Received).

Or Is It relating to a single Item with parts of It missing?

How likely am I to win this claim?

You have photographic evidence of the open/damaged package, so It's looking good so far.