you don't have to get a beatsonic, if just replacing the head unit, if you are electronically inclined, a new separate sub amp will do the trick, or you can skip the sub altogether (but that would be sad)
on the antenna, yes, you will need a new one to receive AM, but at least installation is easy
you can try the dealerships that PureDrifter recommended, but these floor mats are discontinued, so you will need some luck to find a pair they forgot they had - keep a watch on ebay, they turn up from time to time as new, a couple of times/year on the hard colors
what will make a huge difference is what color you need, certain colors are like trying to find gold, and certain colors there are still old stocks of at online dealerships
From what I've read replacing head unit is a real pain.
Nah, in the car stereo world, this is one of the easier ones!