a) I have the 3-way classic series installed in my g/f's car and I can vouch for them. They sound better than any retail set you will find in a store (such as Kicker/Alpine/RF...). From the speakers I have heard, they closely resemble the sound of A/D/S components. Very smooth and neutral (no ear pearcing highs, perfect transition between tweeter and woofer, detailed midrange, and solid, but not overwhelming, midbass down to 60hz)
b) You will have to make a baffle to mount them to because the front speakers are 5x7. (basically an oval piece of MDF with a circular hole for the speaker). If you use 3/4" mdf for the baffle the mounting depth shouldn't be a problem.
c) Every car is different with regards to factory speaker wires. I haven't done an install on my P5 yet so I can't vouch for Mazda's choice in wire. In general, newer cars have better wire. If you do the install and you hear crackling from the tweeters or a harsh fuzziness at loud volumes....replace the wires....if you don't then it isn't necessary.