Hi, Simon. You might want to take a peek in the
Beginner's Area. We've got some good info in there.
First, and in all seriousness, it's very good that you know enough to know that you don't know enough about clarinets. (Something like that.) My suggestion for you is to get a clarinet teacher -- and your teacher should be someone that plays clarinet as his main instrument -- and have him check out whatever horn. Using this thread as an example, the Leblanc Bliss and the Yamaha 250/26 are both decent student horns. Based solely on the feature set, a composite Bliss with a Bakun barrel is probably worth more in the long run. Better? That's a question of taste. I happen to really like Yamaha instruments because they're consistently high quality, even their student models. I've not played a Bliss, but it's got a really nice feature set.
I have serious issues with buying any used instrument for any beginner unless you can have your teacher play the horn OR if you can get a money-back guarantee that says you can ship it back if it's junk. In the link I have above, there are a couple of places listed that do have money-back guarantees on their used horns.
I don't recall if Dave Kessler, one of the posters in this thread and also a dealer, has a money-back guarantee on his used horns, but I do see that he has warranties on some of his used horns. He's also good to deal with -- and I base this on past posts;I've never bought anything from him. You might want to just give him a call and see if he can get something together for you.
Back to the mouthpiece, a clarinet mouthpiece makes as much difference to playing the clarinet as a saxophone mouthpiece does on a sax. The Yamaha 4C is actually a decent mouthpiece, but it's really a "starter" mouthpiece. Spend a few more bucks and get a Vandoren B45 or a Selmer C85 and you'd see a big difference.