As far as I remember, there weren't mini-bosses in Zelda II, so even if we did resort to adapting the Zelda II bosses, we still wouldn't have enough. We'd have to design some original sprites anyway, and if we have to find a good sprite-maker, we may as well to get them to do more original designs.
Basically, because of the attempt to fool people, I'm hesitant to use recognizeable bosses. Even though Seasons has the bosses from the original Zelda, that to me isn't reason enough to necessarily use all old bosses here. Then there's perspective and size issues. So Zelda II can't answer our boss issue. Not totally, and not preferably.
For the first and last major dungeons, the ones people will likely look at, I definitely want original bosses. Sorry about the pressure, but this includes RT's ice dungeon...besides the fact that marioman said it was used in one of the real Oracle games already.
I'll ask bustin98 about spritework. As good as his artwork is, I think spritework might be more necessary here.