For my head unit I went with the Pioneer AVH-X5500BHS. It is a double din DVD/CD unit with a 7" touch screen that has all the outputs for extra monitors and inputs for things like video game systems, back up cameras, and video sources. It does not have GPS built in but you can buy the Pioneer GPS module or use an ap from your phone. I will always either use my phone or the stand alone Garmin GPS I have in my truck. I picked this deck up for a couple of reasons. It has built in bluetooth so everything is hands free, even voice control on my Samsung Galaxy SIII. As such, it also allows me to stream music from my phone on the bluetooth audio input. It is also one of the only decks out that is compatible with Android for wireless streaming of Pandora. When I switch to the Pandora input (after setting that input to streaming in the setup menu) the deck automatically opens the Pandora ap on my phone and I get all of the standard menus and options on the screen of the deck that I would have on my phone or computer like thumbs up, thumbs down, song title, artist, album title, station list, etc. The deck also has full ipod controls and comes with a USB extension cable that I plugged the docking cable into that came with my ipod classic. It also charges my ipod. It has three sets of 4 volt RCA outs (front, rear, sub) and has active electronic crossovers and sub controls built in along with an 8 band equalizer with 5 preset curves and two adjustable curves. It is a very nice unit and does exactly what I need it to do. It has enough power that it will run the stock speakers or some decent 6.5" coaxials or very efficient components until you can get an ampifier. It also has an ap mode if you are an iphone user. With app mode, it will connect by blue tooth and let you display and control any ap from your iphone on the display of the deck. Here is a link:
AVH-X5500BHS - <b>NEW!</b> - 2-DIN Multimedia DVD Receiver with 7" WVGA Touchscreen Display, MIXTRAX, Bluetooth®, HD Radio™ Tuner, SiriusXM Ready, and AppRadio Mode for iPhone® | Pioneer Electronics USA The suggested price is $500 and I paid around $450 for mine and got the shop to throw in the wiring harness since I didn't have the JBL system. You should be able to find it at a stereo shop for $400-$450 or you can take your chances buying online for cheaper and getting a grey market or refurbished unit. Here is a picture of it in my truck streaming Pandora:
As for amplifiers, speakers, subs, and everything else, the sky is the limit. Alot of people on here like the Polk Audio DB series of speakers and will put the DB6501 6.5" components up front with the tweeters either in the dash or in the sail panels on the doors and then put the DB651 6.5" coaxials in the rear doors. You can get the components for $150-$200 and the coaxials for $50-$100. I have swapped my front components a few times. I Have been running a set of the original Polk Audio MM465 Momo components for about 12 years and then decided to try some Image Dynamics CTX65cs components when one of my Polk tweeters went south. The ID mids and tweets sounded OK but I think the crossover was making the tweets sound a little harsh and the Polk mids had a warmer sound to them so now I'm running the Polk MM465 mids and crossovers with the ID tweeters in the sail panels and they sound great. Not being one to quit messing with my truck, though, I will be going to either the Hertz High Energy HSK165 ($450) or Rainbow SLC 265.25 Kick (can get a like new set for $330...MSRP $640) or possibly Hybrid Audio Technologies Clarus C61 ($630) in a few days and I'll switch over and run the mids and tweets active.
There are a whole bunch of amps out there and the advice I'll give is to stick with well known name brands and get something with enough power that you won't starve your speakers or subs. I've run Alpine amps and liked them. The Alpine V Power series is their entry level and actually has very good sound quality for the money. I'm currently running a Boston Acoustics GT4100 reference amp on the mids and highs and a Soundstream Rubicon RUB1.1000D on the subs. The new rage is class D amps because they have a smaller footprint and run cooler and more efficient than class A/B amps. I'm fine with that for subs and my Soundstream is a class D amp but I still think the sound quality on the class A/B amps is better for mids and highs and the amps tend to sound warmer and more rich...just my preference.
You don't mention what cab configuration you have but there are only a few options on enclosures for each so you should be able to find several threads here on the forum about that. Alot of people are going with JL Audio Stealth Boxes. I'll admit they do sound really good but JL gets a premium for those things and I don't think they are worth what they want. Of course I am saying this because I am comfortable matching subs with enclosures and even building enclosures when I need to. If you're not, either buy the JL Stealth Box or a decently pre made enclosure from somebody like supercrewsound.com or have the local stereo shop build you one. Then you just need to decide what size and how many subs you want.
The last thing I'll recommend is good high quality wiring and RCA cables and sound deadening. You need to have a large enough main power cable that you don't choke your amps. I use 0 awg from my battery to the distribution block then 4 awg to my amps. The ground cables need to be the same size as your power cable. Use RCA cables designed specifically for car audio and of decent quality. This will reduce the chances of introducing noise and will give better sound quality.
Sound deadening is important and can increase the SPL in your truck by 3 db just by reducing the amount of noise in the cab that cancels out the music and keeping the metal from ringing. Do some sort of sound deadening even if it's just throwing a few CLD tiles dow on large areas and then completely cover and seal your doors. It's probably the place that you will gain the most bang for your buck. I spent $300 on sound deadening materials and that $300 made more difference than any other $300 I spent in my system.
Of course, this is purely my opinion and you'll get alot of those on this forum.