4500 is a lot of money, especially since this doesn't include installation.
I've probably spend about 4500 on 3 different setups, first two were on the cheaper side (price wise) the last one has cost me quite a bit...regardless.
The Clarion 9255 and Pioneer P9 combo are both great HU choices, but do you need to run a 3-way active front stage, plus sub, or is two way going to do it for you? If 2-way is fine, then the 880 is a very nice choice, I love mine to pieces.
Amps: You don't need to buy Zapco to achieve good sound quality, just because you have that 4500 limit, doesn't mean you need to spend all of it, when you come in under that budget (you will) invest it into something. There's a lot of great amplifiers out there that might not have the same stature as something like Zapco, but that doesn't mean they're any worse, my personal preference is Linear Power, but they're hard to come by so unless you want to search for months, going with the bigger mainstream brands isn't bad. The Memphis amps are supposedly a great deal for the price. Alpine's PDX amps are a bit pricier than Memphis, but supposedly very well done. Also, apparently Kenwood's new eXcelon amps are a great bargain in the performance/dollar ratio.
Front Stage: I'd run active, single best thing I ever did for my musical experience. Don't be afraid to try out drivers you've never heard, I bought mid midbasses on a whim and fell in love with them. There are tons of great home audio speakers out there that work very well in car, DO NOT be afraid of the 8ohm impedance either, IMO it doesn't mean much to me at low power levels compared to most people's substage of 1kw plus, where something like 500 or 1000 watts is audibly different.
Speaking of substage, have you thought about where you're going to put it? My guess is you don't have a lot of room, I would think 2 8s or 2 10s would be sufficient, but then again i'm not a bass head so 2 10s to me is just fine.
Also, sound deaden the crap out of you cab, this will make a huge impact on quality. It'll help block out road noise, engine noise, and lower break-ups that could be caused by panel resonation. Add in some closed cell foam to that as well, it helps absorb unwanted noises as well.
Hope this helps some.