Idk about AUS specifically, but you can generally get a 5TB external spinner for $90-$100 that you can Velcro to the top of your NUC. Minimal size and works great, runs direct off USB power. No way would I pay for a 8TB SSD in there.
4TB SSD bottom out around $150 (US again) and would be perfectly good for Plex (don't need quality for that), but again it's a cost issue.
I'd also consider something larger than a 512gb m.2; that's a lot of wasted space and 2TB-4TB m.2 is just as cheap as SSD.