Ok, I'll slap my advice in:

Headunit; For a decent double din headunit that is worth getting, and the gps isn't woeful. You'd want to spend around the 800-1k mark no doubt. Sure you can get cheap ones for 300 or less. Odds are the interface is hitler, and the gps works about as well as a dyslexic reading a map.

Speakers; At no point in time is it worth buying coaxial speakers for your car. Period. Bar if you have passengers often, and they want to listen to music. The difference from 2way to 3way+ coax is very little, and the sound difference would be barely noticeable at best. However, going from 2way to 3way splits IS very noticable. Why? Instead of there being two seperate speakers doing the work (tweeter and mid) you have three. So, each speaker works less on certain frequency ranges, meaning they can pump harder in their own range. Although, instead of the difference being something like $10 (ie, 2way being $50 and 3way being $60) the price difference of 2way to 3way splits is a few hundred. (ie, $300 to $600)
But either way, you don't need rear speakers in your car. Scrap any plans for rear speakers, and put that money towards the fronts. Why? We use the example of a live band. When you go to see a band live, you don't sit in the middle of the band, and listen with the music around you. You don't turn your back to the band. You sit forward, facing the band. your ears are designed to listen to things from the front and side more or less. So, it doesn't make sense to have music coming from the back of the car, either.
A good sound stage in a car is having the band sound like it's playing on the bonnet. Hard to achieve, but around that is good enough if you're not aiming for top points in sq classes.

So, what you want to do, for the best results for your money. Get some splits. A good set is something like morel maximo 6.5 splits, crescendo opus 1, etc. Both around the $250-300 mark. Both very capable.
Next thing you want to do, is install them solidly. Now, what you want to do, is get some mdf, cut it out to suit your door, and so the speakers sit in there nice and solidly. Then you want to screw the spacers to the door, so they are solid. And screw the speakers to the spacer. Make sure to paint the mdf spacer with some form of paint, so it doesn't get waterlogged, and die within a few weeks.
Lastly, and very important. Look into dynamat. What it'll do, is help with the bass coming from the speakers, and make your door more solid. Reducing vibration/ rattles, increasing speaker performance, and just generally making them sound better. Including the satisfying thud you get when you close your door.
Just remember though; when choosing speakers, it is ideal to listen to them before you buy. Not everyone has the same set of ears. Some people love speakers with really peaky high ends. Some hate that with a passion, and it makes their ears bleed. Some love speakers with muddy sounding low end. Other hate it with a passion as well. It's all preference. Granted, it's hard to hear a lot of speakers that are well setup (sound boards aren't a good example of how they sound).

Amp; Realistically, you can get anything for an amp. Although, cheaper ones (ebay spec) can tend to lie about their performance, and be horribly in efficient.
What you'll want to do here, to keep costs down, but still get good performance, is get something like a jaycar 4x100 amp. A nice 4channel with a decent amount of power. Slap your front speakers off the first two channels, and save the last two for the sub (probably helps to make sure you can bridge those two channels). With the jaycar 4x100, you should see around 300wrms at the sub at 4ohms roughly there, which is heaps.
Avoid boss, powervox, kingwood, etc etc. like the plague

Sub; Now, this option is another one you'll need to be careful of. If you do the above with a 4channel amp, you'll need to get one or two types of sub. Either a single 4ohm voice coil. Or a dual 2ohm voice coil. Why? Most amps when bridging channels will only be stable at 4ohms. And a sub at single 4ohm or dual 2ohm will be able to wire to 4ohm, and keep the amp from frying, or catching fire. If you get a dual 2ohm sub, you'll need to wire it in series. What that means is amp channel 3 positive into voice coil 1's positive. Then a wire from vc1 negative to vc2 positive. Then a cable from vc2 negative to amp channel 4 negative.
I would suggest the sub I have, however it is the wrong setup (dvc 4ohm).
Although, since you want to look into an underseat sub(s), you won't need an amp, and they'll be fairly simple to install. Realistically, if you have two, it will be a bit louder, but not hugely. It'd be easy enough to install the two as well. And they'd sound fine, as they'd be getting the same signal. Unless you gave one like 5 meters of signal wire, and one 1 meter.



Now, I'm going to say; The mainstream brands (ie, alpine, pioneer, kenwood, etc) aren't anywhere near as good as you'd think. They are all entry level brands these days. Entry to mid. And are well outclassed in their price by many lesser known brands. A good example is something like the alpine type s being dominated by the shok triton sub. Which is about half to a third of the price.
Reviews and feedback places aren't the best places to find info on speakers. Generally with feedback and reviews, 90% of the reviews are negative. the other 5-8% are paid for, and the last percentage are actual good ones.
Basically, don't use them.
Personal reviews are better, but still not great. Your mechanic might love his 2ways, but they'd be not much better than stock, and would be destroyed by a well setup pair of splits in the setup above (solid install and dynamat). Although, keep in mind; shit speakers in a great install will sound better than amazing speakers in a shit install.

In car audio; Install is 80% of how your car sounds. If you cheap out on the install, it's highly likely your install will sound rubbish. Gear choice is 20%, but still very important. You get what you pay for. Except with mainstream products. They have extra cost for their logos

Quote Originally Posted by cbauto View Post
Changing HU and speakers doesnt really count as a setup.
300buk splits are not good speakers
On the contrary, you can get some bloody good $300 splits.