Hi and welcome to SCS!
You are doing postcards? (no envelopes) How many?
Inks will tell you if they are water based which you dont want...
Archival was a good suggestion and they are widely available. Craft stores, stamp stores, Blick art, online-they are from Ranger...I would think you would not need to get a re-inker for this once off. But black is a good basic color to have in general if you are a stamper so I would if you are getting that color. Just know their pads might leak a bit and be a little messy as recently reported here.