Metamorphogenesis shouldn't be too difficult to find. I'm sure The End will restock it, since it sold out within a week. If you see Esoteric's first two releases, do not hesitate to buy. They are very hard to get a hold of, apart from eBay. The Pernicious Enigma is usually on eBay, but I've never seen Epistemological Despondency on eBay, but I've heard it fetches a rather substancial sum. That being said, I've heard that the first two are being re-released, so hopefully that will make them easier to find.
Primordial, one of my favorite bands. I'd say the eseential releases are A Journeys End and Spirit the Earth Aflame. A Journeys End is much darker and bleak, has much more atmosphere than any of their other releases. A comparison to Agalloch would not be far off, but Primordial are much more black metal than Agalloch. Spirit the Earth Aflame is the most epic release, and was the best album of 2000 IMO. The singer really hit his peak on this one. When I say "epic" I don't mean viking metal. This is pure pagan metal from Ireland, with that Celtic flavor (no, there are no tin whistles).
SO it depends what you want: darkness and atmosphere or paganism and epicness.
Imrama (the debut) and Storm Before Calm (the latest one) don't stand up to the other two IMO. Both are much more black metal than anything. Still good albums in their own right, but they ar eclipsed by AJE and STEA.