IF you put some decent splits in the front, I wouldn't worry too much about what goes in the back.
BUT
I have a 98 Foz and I just stuck in some 180W BOSS 4" 3 ways I had sitting around.
Sounds heaps better than stock.
The 4" speakers didn't quiet make a tight seal between the speaker basket and the door, so I grabbed some weather tape from Big W for like $3.98 and used it to create a tighter seal.
Luckily the rear windows don't go down too far, so you probably wont have a problem with the rear speakers.
Alpine is awesome, and some 4" coaxials probably won't cost much at all.
Hope that helps