Who said HGSS didn't have a lasting interest? I personally love it and can't get enough of it. If you really, really don't like gen V stuff, then I suggest just getting HGSS now, and maybe waiting til after BW is out for a while and deciding from there. I'm playing White right now, and it's really great, and I didn't know any of the new features that would be in it because I avoided spoilers. But my best suggestion to you would be to just get either HeartGold or SoulSilver and give BW some more thought and wait a while after it comes out. Besides, by the time it does come out, I'm sure you will have saved up enough to get it, and maybe your interest will have built up.
About the Pokewalker, I'd just buy a used copy that doesn't include it. I never use mine, it's not really worth it. It's fun the first few times though, but nothing that great.