You will nearly always find frequency response curves and impedance curves for any reputable driver - either at the manufacturer's site or at the dealer's site (or both).

For more-than-2-way systems, sticking to not only the same manufacturer but the same family or line within that manufacturer will reduce your problems for the woofers and mid-woofers and their so-called "voicing" will be very similar. There is no particular advantage or disadvantage to mixing brands when it comes to tweeters versus the lower-frequency elements.

As a concluding note: yes, you can save money by buying a kit, but you cannot make a set of cheap drivers sound like a set of expensive drivers - paradoxically, that is exactly what some of the speaker companies do through their highly engineered systems.