I've heard very good things about resold HGST Helium enterprise drives and can be found fairly cheap for what they are on eBay.
I'm looking for something from 4TB upwards. I think I remember that drives with very high capacity are more likely to fail sooner - is that correct?
4TB isn't even close to "very high capacity" these days. There's like 32TB HDDs out there, just avoid the shingled archival drives. I believe the belief about higher capacity drives is a question of maturity of the technology rather than the capacity. 4TB drives made today are much better than the very first 4TB drives we made a long time ago when they were pushing the limits of technology.
Backblaze has pretty good drive reviews as well, with real world failure rate data and all.