We could all argue about the quality of Polk, Infinity, Boston, until the end of time. My suggestion is, find a local car audio dealer after you have found the proper sizes of speakers. They're going to try to sell you some speakers...and you might even get a good deal there. But here is the key: Have them play the Polks, the Infinitys and anything else you want. Listen with your eyes closed and ask the sales guy not to tell you which brands are playing. You'll hear the difference, and that is the best way to make your decision. No speaker is designed to last forever. A lot of the newer speakers are built to "marine application" specs anyway, so you're going to get a good speaker virtually any way you go.
As for the rear speakers, I've seen guys do some modification and put tens in there. You can put virtually any speaker you want in the rear...plenty of room. The standard size is something like a 6x8...but they ****.
Snowman74