I would recommend a DID ERT 520 series chain. O-rings are more expensive, and on a 125, they do rob you of about one HP or so (so others might atest to this). If you do a lot of trail riding then an O-ring chain might be good. They do last longer, and you don't have to tighten your chain as much. But as they get old, they tend to kink. And you always have to make sure you care and clean for it properly, like using "O-ring safe" lubricants. Another tip, you should always replace your chain and sprockets together in complete sets. It's the only way you'll get the longest life out of your chain and sprockets. Putting a new chain on old sprockets will lead to premature wear. I have no experience with X-ring chains, so I can't comment there, but they are about just as expensive as an O-ring if not more.
For tires, I would recommend a good intermediate tire like a Dunlop D756, it's just a good all around tire. But I hear a another tire like the Maxxis IT, is similar in design but is supposed to last longer than the Dunlops.