Well, with RAID6 I'll get a performance upgrade anyway, right?
Idea of buying bunch of used 2Tb drives is tempting, but I still have doubts about it.
Performance upgrade from what? A single mirror pair? A single drive? Maybe, depending on the access pattern. Performance of an array depends greatly on how it's accessed. That's one reason there are so many types. There is no one size fits all. If you want to store movies for simple playback on a single client, the performance isn't an issue, any of them will do. Start running 6 clients, it won't keep up. At least, it doesn't for me. Basic file serving, probably ok most of the time. VM/database storage? It will likely cause issues.
The used drives are a good cheap way to get started up. If you thoroughly test them, they are fine. If you try to blindly use them, you might run into problems later. No different from new drives. I had a similar failure rate with those vs new, so have most people. The seller took the return back with no issues, I had one DOA. The full tests take a few days to run. There are no shortcuts here. You should be doing it with new drives if you like your data. Particularly with ZFS and somewhat frequent checks, I like them. If whatever filesystem/RAID you use can't verify data, they are probably about the same as anything else really.
What is your use pattern? How is the server connected? Local processes hitting the array? What are your priorities (data integrity, speed, space efficiency, etc).
If you're not sure about those, one option is to pick up say 4 of the used drives, make an array and copy your data to it. Now hit it with all the things you want to do with it, and a couple other things for testing. Watch various stats like I/O wait, speed per disk, speed per client, IOPS, etc... Then try with a different style. Maybe do RAID6 first, then mirrors, then raidz and ZFS mirrors, then md or snapraid. Just get an idea of pros/cons of the types for your needs. Then consider expansion, error handling etc before deciding which way you want to go. That's how I did it when I got started, it was helpful to see the various options in action.