To me it depends on what you're doing. If you're buying eBay or antique razors and fixing them up I'd buy a bevel setter before anything else. If the bevel isn't done correctly, all the other stones are pointless. If you're maintaining razors you already have, you don't really need anything else, but I'd take a look at naturals if you feel the itch.
I use a Chosera 1k, but there are other options. The shapton pro 1500 gets good reviews these days. I have never used one, but the king 1k is said to be a good stone, it just needs to be soaked prior to using and needs to be lapped more frequently.You have a bevel setter in mind?