[Spuds]

Yes, headers, overpipe, and front pipe are all typically sold individually and will bolt up to stock versions of the others. There are some exceptions like Ace headers but those are uncommon.

You will probably need new gaskets each time you replace exhaust parts so if you plan to do more, you may want to do it all at once.

Upgrading the overpipe is not going to give you any performance improvement by the way, and may cause your sway bar bushings to melt without a heat shield.