Hi Onlyoblivion,
1) when you say antique nickel do you want a nickel matte, or a darker nearly gun metal type finish? Antique nickel is a broad finish in the realm of hardware, so most likely it will be difficult to match this finish across multiple suppliers, unless one supplier has all items you are looking for.
2) If it is a center bar buckle, then it can be used 2 ways. 1) what Thor has said. 2) to act as a keeper to hold the leather down against the strap.
3) Most likely you will need a separate setter for each size. Maybe an anvil can be interchangeable for the bottom piece, but the stick to peen over the metal will need to be sized correctly.
4) Again Thor provides a good example.
Thanks!
- Hugh