I'm just now playing the Breen missions, and i see you get a unique deflector dish amongst other things.

I had two questions:

1) Can I obtain these in Mission replay.
2) And though unique, can I discard/replicator a lower level version and replace it if I can obtain these as in question 1?

You can obtain everything but the Breen officer and the emblems through mission replay.

You can only have one unique item. I am not certain if you have to discard a lower level Breen item to obtain the higher level one. If you do, it will tell you when you try to complete the mission that you cannot complete it because you can only have one unique item.

But I CAN discard it and get a higher version? And thanks for such a prompt reply.

Yes. There is one for each level up to XI. The sets only work if every piece is of the same level, so if you mix X and XI, you miss out on some of the set features.

It is also possible to have different levels of 'Unique' item set... (e.g. a Mk IV, and a Mk VI, but not 2 x Mk VI) this means you can use an existing 'Unique' set while working on the next higher set after you Rank Up...