hey, i just recently sold my md421's, and the only thing that i miss it on is guitars, kick drum, and bass cab
(definitely get one) for toms, if you can look into the audix d2's, i like them better than the 57's and the 421's (you might as well just use the 58's you have until you have a bigger budget)
i think you have the right idea with what you are planning on doing with your mics, but i see one problem. i personally think that you could be looking for mics that are different that what you already have.for vocals, I HIGHLY recommend using your sm-7 right now, going to a studio really listening to mics until you really know what you are looking for, and get a quality LDC.
good luck with your search, i'm going through a similar phase right now