Welcome to the Antique-Shop forum, Briaeros.
Maybe one thing to look at/for would be a ring that comes with a genuine (not some computer print-out) & verifiable certification/appraisal of the antiquity and value of the item.
There are many, many reproductions/fakes/duplications on the market for items such as you mention.
Generally speaking, most rings of that era are cast metal, which is simple for modern metalworkers/jewelerymakers to duplicate (some are even made from actual impressions of antique rings) &/or counterfeit.
Having such an antiquity professionally certified can add up to $500 in additional costs to your purchase.
Keep in mind that what you're seeking is considered a piece of antiquity (a catagory that sort of supercedes the term 'antique'), and expect to spend some serious money, especially for rare metals (& consider the fact that even the 'experts' make mistakes).
Perhaps finding a reputable dealer in such antiquities would be a route to go, in the event you don't wish to do TONS of self-education about such items.
Anyone buying sight-unseen (in person) antiquities puts themselves at high risk for buying counterfeit/less-than-authentic items, IMO.
Best of luck in finding the authentic item you seek, Braieros!!!