I usually thrift pens so I have some considerations for you consider

Gold nibs are usually bouncier in my experience and write a smidge wetter.

Nib thickness not nib size is what you’re looking for. Japanese pens in Fine would write a bit thinner than a western fine

Check for nib alignment or damage. If the nib tines are shreked I’d consider if I want to shell out extra for repairs.

Schimdt? Jowo and Bock. Pens with these nibs are not indicative of brand but they’re good nibs. Jowo and Bock can be swapped out iirc for same nibs if it needs replacement. Schmidt nibs are not easy to swap out.

Filling system. You’ll have to go with the expectation of the sack of a lever filler pen is not good condition for example. Would you need some extra tools to check it. Is it proprietary? Etc

It’s not gonna be pristine so temper your expectations.

Ask your self, is it worth it? Is it reasonable? Is the price of the pen the same as a new one? What would it cost to get it restored? What/who are my options for restoration?

Additionally you could try out the budget friendly FP’s to feel out your likes