iPad Air 3rd vs 5th Gen for PhD Reading + Best Refurb Sources?

Hi all! I’m starting my PhD and want an iPad mainly for reading papers and light annotation. My main work machine will still be my HP laptop, so the iPad is mostly for portability and comfort.

I’m torn between the iPad Air 3rd and 5th gen — is the upgrade worth it for mostly reading, or is the 3rd gen still fine? Also, any trustworthy places to buy refurbished iPads (UK preferred) with good warranties?

Looking for advice on performance, battery life, and Apple Pencil support. Thanks!