>>4
If you just want SOTN, I would just download it. Saves you the wait.

Castlevania II for game boy was pretty good, despite being a bit short, but the other 2 castlevanias on the Gameboy weren't all that great. Depending on the version of II you play, your subweapons will be different.

Did you ever play Castlevania Bloodlines for Genesis? Depending on who you ask, it's better than IV for SNES.

Dracula X/Chi no Rondo/Rondo of Blood is arguably the best traditional platformer one. It's for the PC Engine, and also included on the recent PSP game revamp of it. IIRC, Symphony is also included with the PSP game as well, so you might consider getting that to get all three. There was an SNES port of it, but it skimps out on some of the good stuff the original had, like the extra character and, IIRC, the branching level paths. So, if you just want SOTN, you can just download it, but if you want it and this game, you might as well go this route.

GBA is also good too. Circle of the Moon is my favorite non-olden platformer. There's a number of extra "modes" to play after you beat it as well, but you have to beat them successively (Normal>Mode1>Mode2>Mode3>Mode4). Aria of Sorrow is damn fine as well. The bonus character you get after beating it is great. I highly recommend you NOT play Harmony of Dissonance. It's really not that enjoyable outside of using 8-bit simon in the boss rush. That itself is fun, but getting to be able to do that isn't. At all. And the bosses suck.

As for DS, I honestly do not recommend Aria's sequel, Dawn of Sorrow, simply because it really isn't as good. The extra mode is sort of cool, but it's done much better in Portrait of Ruin. If you're going to play Portrait, then play it after playing Castlevania Bloodlines and Dracula X so you can catch some of the references. Portrait is a bit thin and not as fleshed out as most other games, but it's still pretty good and has the most extra characters I can think of so far. In addition, it has really brutal hard modes. The newest DS game is a bit different from standard Castlevania action, so I would recommend you play that after some others and regain your footing into the franchise. It's not that it's bad, it's just not quite played the same as previous games.

As for the "hook", it just depends on what your preferences are. I think Circle of the Night is the best well-rounded Castlevania to date that incorporates the newer exploration-type maps while keeping the old platformer flair, without any mandatory new fangled stuff like turning into animals (although this itself is an old one, technically), having an AI partner, or having your weapons solely being magical. On the other hand, Aria of Sorrow is different from the conventional stuff, while still being enjoyable in its own right and the extra character is very much like the old castlevania heroes in his own way, and getting him is enjoyable, too.

I recommended a few old ones, but that was just for the sake of "Have you played this yet?" If you just want the recent ones, then don't forget the GBA. Unless of course you have a DSi, in which case, I recommend buying a Micro or SP, depending on your preferences. GBA stuff isn't that expensive anymore, and since you seem to be willing to buy stuff, they shouldn't cost too much, compared to recent stuff that is. I don't think you should have too much trouble finding either, be it in a store or online.

Happy hunting.