Yeah, story and game engine things tend to be pretty separate for the most part. The main impact the game engine has on the story is how the story is to be told, which is mainly connected to the limitations of whatever engine you're using. But yeah, it's usually not a bad idea to get the engine stuff established first using more basic assets before increasing the complexity, because your game's engine is its heart more often than not.
As for BMW, it definitely has its flaws, but it isn't a horrible game, certainly. It's can be pretty fun when the Battle Masteries aren't completely stupid (there's a few of those, yes. That one stage where you have to position Ciel in just the right place in order to keep that one girl from being killed, I hate you so much). Plus the item/skills/stats system suffers from being somewhat confusing and not explained well enough, and the lack of points and how certain points are shared between certain sets of things can be a bit frustrating. However, it has its fun moments, and has the potential to be really fun if it's cleaned up and the flaws are addressed.
And I can serve as an artist if we absolutely can't find/afford anyone else. I don't think I'm that awesome at it, but I have the benefit of being free of charge, and it's better to have a mediocre artist than no artist at all. Still, if we can find someone willing to do this that's far more experienced than I am, that'd be good, but at least I can serve as a fallback if all else fails. I'm not a spriter though (though I can do some recoloring and minor tweaks if need be), so we'd still need that even if we end up using me as an artist.