I maintained my first straight razors for years with just a barber hone and my strop. If you can find a nice specimen it will surely do the trick, and they are usually quite affordable.
If I was to do it all over again however, I would have started off with a coticule. They are great finishers and are very versatile. Which means that as your honing skills progress you can learn to do more work with slurry if need be.
The advantage of buying a new synthetic naniwa 12k or 3k-8k combo stone is that you don't need to worry about it not performing as it is supposed to, they are all the same whomever you purchase it from.
Should you decide on a natural stone or a barber hone, buying from a knowledgable vendor will ensure you get a suitable stone for your needs.