I’d also agree with the front over a complete rap around system. My last truck I did this (sorta). Instead of dumping a bunch of money into speakers for the back doors, I just popped in some two ways for ‘fillers’ in the back doors and ran components in the doors with a pair of ten’s under the back seat.

Components are the way to go for the front. You could probably get away with just running a pair in the front and a couple 10” subs in the back somewhere. Most car audio guys will recommend installing foot pods down by your feet that direct sound up and into the cab. That’s not real practical for me as a truck owner. The speakers would be too close to muddy/snow covered boots, but it may work out OK for a car that lives in a sunny / fair weather environment.

X-overs can be mounted wherever it is convenient to do so. Sometimes you can mount them in the doors behind the speakers, but most of the time, installers will mount them where they are accessible. Sometimes under a consol. It really depends on the vehicle.

I really like Alpine head units. I don’t much care for anything else Alpine makes, but their HU’s are outstanding.

Speaker choice is not any different than home audio. It all depends on your preference. MD Quarts are great speakers and get a lot of praise, as well as Focals. My last set of components were CDT. They sounded incredible. I’m not sure what they have out now, but they’re worth looking into. http://www.cdtaudio.com/car_audio.htm

For amps, I used Phoenix Gold. They make some kick ass amps. My subs were Sound Stream Van Gogh series, which are a very musical SQ sub. But as with any sub, it’s only as good as the enclosure.

You’ll also want to run an Optima Yellow top battery. Some guys will tell you to run a capacitor, but I prefer going with a good battery over caps. In either case, you need one or the other or your lights will dim every time the sub hits.

And of course you should install dynomat everywhere you can. Definitely around you subs and speakers. But if you can pull all the carpet, door liners and roof liner, installing dynomat will dramatically improve sound.