Hmmm..........
Sounds like you haven't comeup with a plan yet. You need a plan before you go anywhere.
Sit down and decide what features are important to you (ie, music, gps, obdII, internet, games, backup cam, etc, etc).
Next, you need to draw up a shopping list of components you will need to achieve your goals (ie: MB, PSU, LCD, bluetooth, etc, etc).
Now you have to decide on an interface, or "how you are going to control" your system (ie, RR, Centrafuse, etc, etc) and a skin to intergrate it all (lsx void is my fav, there are multiple others available on the site, or you can make you own)
Once this is done and running stable on the test bench, you need to install it in the vehicle. Lots of choices here. This is almost back to the plan stage as you have to decide if you will keep the HU, what amp to go with, speakers, woofers, etc, etc.
At any rate, this is not a do it in a weekend project (unless you're planning to put a laptop in running through the hu with an LCD taped to your dash somewhere

)
If all this seems a bit overwhelming, just relax, It's not that hard. There's just alot of steps that involve a fair investment in time and effort. Lot's of reading on the site for you to sift through to decide what you want and what you can achieve at your skill level.
You could also go with a "more or less" complete system like those offered at teh MP3CAR.com store..........
Good luck, and read/search
alot!