it depends what you'll be using it for.
marty is good for certain things, i got my audison srx3 amp from him. As for my splits, i got a very good deal from american ebay. i also got my wiring (stinger) from marty.
As for the screen, it depends what you want. ebay is decent, but yeah warranty is a bit of hazy area. I'm putting in a carputer atm and i'm going to get a 7" lilliput touch next week from ebay to navigate it all. Go for the lilliputs, they seem to be pretty good quality (except the indash 7" touch screen which i've heard horror stories about). If you dont need a touch screen then go for one of the standard ones, they go for alot cheaper than the touch screens.
And also what one of the other guys said here ... rear fill is kinda pointless if your getting subs, as everyone will tell ya, your ears face forward. If you've got pretty good splits, putting 1/2 dodgy rears is gonna screw up the sound a bit. I guess if you must have them you can always fade them out if your running a headunit.
and a decent mp3 AV setup is going to cost you about 1g at the least.