They're putting out so many new supplements at the moment (Gondor at War, Scouring of the Shire, War in Rohan, Quest of the Ringbearer, with more on the way) that you wouldn't be short of material just collecting the "current" books.
You might find some of the old supplements, the War of the Ring massed-battle rules, or the Battle Games in Middle Earth magazine cheap on ebay or in a charity shop somewhere, or lurking half-price at the back of a games-shop. If you like scenarios or looking at photos for painting inspiration could be a good purchase. I'm sure you could adapt most of the old scenarios to the new rules.
If you want to get into tournament play, watch out for this one coming back:
https://www.games-workshop.com/en-GB/Mi ... ok-EN-2020