Before buying speakers, I'll expound more on the points raised about installation and acoustic treatment. Aside from the latter, which is still a lot more than just putting Dynamat everywhere, you have to work with the speakers to get around the fact that 1) you're not sitting dead center and are too close to one side of speakers and 2) there's a bunch of things - steering wheel, center console, etc - sitting between each speaker.
1) You have to angle the speakers, especially the tweeters (and midrange if it's a 3-way front+sub) to minimize reflections off its dispersion pattern on the windshield while putting the vocals as dead center as possible for both front seats. Fine tuning will be through electronics. Back in the early to mid-2000s there was a local autosound competitor using an Alpine F#1 Status receiver-processor (the one with the control panel that expands to 1.5 DIN when powered up) and $50 woofers and tweeters, and his system, properly installed and electronically tuned, actually won over others with better speakers.
2) You need to get a receiver or processor with the following features if you really want to fine-tune the system : 3-way (2-way + sub) processing for time alignment and crossovers. Personally, forget the EQ specs, these are the most important. Right now the cheapest is the Pioneer 80PRS at $349; if you don't want to hack up the current styling of integrated audio systems, Alpine has an integration processor (uses speaker-level inputs) with the same features for around $600. The downside is you need to hook up a laptop to tune the settings.
a. 3-way crossover means you can split the sound coming out of it to send a different signal to the tweeters, midwoofer, and sub.
b. 3-way time alignment will only be really useful if you have a 3-way crossover network. Most receivers only delay Left vs Right and Front vs Sub - what you will need to do is delay the driver side tweeter, driver side midwoofer, passenger side tweeter, passenger side midwoofer, and subwoofer relative to each other. If all of them arrive at your ears at the same time (after installing/angling the drivers properly) then you can position the vocals dead center and have a relatively "normal" soundstage all across the dashboard. On the Toyota Vios/Yaris sedan, the flat, center-mounted hump allows for easier management of reflections off the dash,and I've listened to one car where the drums sound like they're on the hood.