You need to mention what you need and what car you have.
Don't forget the little things like woofer enclosure + wiring + labor. You have a small car, thus I would suggest stick to a small setup that focuses more on quality than loudness. JBL GT5 12", JBL gto 10" which ever sub you find ( You can get a pioneer/kenwood sub if you want headaches or if you love SPL ), jbl 608c components ( These are 6.5" drivers so you need to check what size speakers you have in the front doors ) and a 4 channel amp that can pus atleast 250+ rms bridged @ 4ohms.
There are alternatives to the jbl comps and subs too, alternatives that sound as good but may cost a little more. None the less, you can also opt for components by pioneer, sony and kenwood too but those are bright speakers and you wont feel like listening to them after an hour.
You can put your total cost to approx 30k for a good setup like the jbl or 25k for the basic setups consisting of pioneer, kenwood and sony drivers. Remember, I'm telling you about the brands that are readily available. If you want to look for specific brands, then you might even fall across the like of jl, fusion, infinity, audison, Nakamichi etc. But these all will cost that little bit more. And that little bit is a never ending expenditure when it comes to audio builds.
You either buy regular 4" coaxials that will distort at high volumes or you can simply spend around 30k+ for some 4" drivers that wont distort at high volumes. Its upto you. These are 4" coaxials you are talking about. The cheapest and most brittle type of speaker. I've tried alpines, pioneers, sonys, BA's and 4-5 other makes ( All of these cost under 10k for a pair ) and they all had a limit. You cannot expect these to produce bass and neither should you expect them to sound ultra clear at high volumes. 4" drivers are only meant to full fill the needs of listening to music. I would suggest you hit the junkyards (shoba, shershah or whatever is nearest you ) and look for some nice 4" drivers in the dashboards that get sold at these places. You might find some great coaxials under those speaker grills.
Depends on what your needs are, especially if navigation is a must. If not, the new pioneer AVH-X8750BT is a great unit, one that is being used by many pw'ers here. I use the AVH-X5550BT and that is a great unit too. Tons of features. You can also look at the 5750bt aswell.