If you don't have or don't plan on adding a sub, I'd buy a mid with as low as frequency response as possible and up to around 3,000Hz on the high end. Then, just pick a tweeter that will play at least below 3,000Hz.
(3,000Hz is just an example, just used it because many pre-made x-overs are in this area)
If you have a sub or plan on adding one, you can easily get away with a mid that will only play down to 60 or 70Hz as you'll cut your sub around 80Hz.
From my personal experience, I've found it's difficult to find a mid that has both excellent mid-range and mid-bass. They tend to be skewed towards one or the other (well, in my price range anyways).
That Peerless mid in my recent cheap component post is a nice, bargain choice for a system with a sub. BTW, I'd recommend silk/fabric tweets as metal can sound harsh/shrill, unless those are your tastes.
