70s Leader Precision w/Shimano 600 (road), IRO Rob Roy (Fixed)
Cheap cogs 16T and under = Shimano Dura Ace.
I got a 17T Formula cog to start off with on mine, and that worked until I rode it into the ground and gave all the teeth a nice curve on it (chain wear through lack of maintenance... don't do that). It's also still stuck on the hub, clean the hell out of your hub threads and use grease when you install your cog and lockring. And use a chainwhip or rotafix to get it down tight before riding off.
E.A.I. Deluxe cog, if you're switching to 17. Dura Ace doesn't go up to 17.
Usually, if you don't mess with it much, and put it on right in the first place, the lockring can be reused. Though "cheap" ones are made of aluminum, and that's not ideal in a situation that involves threads and possible rounding out of the lockring notches. I recommend Dura Ace lockring, which is hardened steel, you can confidently reuse those.