(1) They won't be as good as Vifa, Peerless, Morel, etc. but will be a significant improvement over drop-in coaxials. I think they'll do a good job in providing what you're looking for, especially with some good amplification.
(2) A crossover frequency between 2-3Khz would be best suited for these drivers. The ones you see on ebay are probably all in the general neighborhood. I wouldn't lose sleep about it. I'd look for one with a tweeter attenuation circuit which would help with tuning.
(3) Not sure, but I don't think those tweets are anywhere near 3" tall, maybe around 2". In any case, there's at least a good 1.5" behind the door panel to work with, and you don't have to mount them on the plywood.
