Regarding your audio upgrades I'd recommend getting a head unit that's not too deep, as I've had some issues with longer/deeper units before I got my Blaupunkt one.
So, Ideally one without a CD drive, as those add a lot of length and therefore might interfere with the wiring loom in the back.
You'll probably need some spacers for aftermarket in-door speakers, and the dimensions you're looking for are a 165mm diameter and around 18mm in thickness.
For example I myself got a set of Hifonics Vulcan VX-6.2C speakers, which are a two-way set with an external crossover unit that I just taped to the inside of the doorcard, and the Hifonics tweeters perfectly fit into the OEM triangle tweeter units, at least if you carefully adjust the metal prongs that hold them in place.
They sound quite good by themselves, but definitely lack bass.
But I guess folks that want to upgrade their audio system are also going to get a standalone subwoofer, so... yeah.