As someone with a ridiculously high end install in my car (courtesy of a friend's shop who wanted to use my car for a show piece), I first thought "man this is really pointless, having all this gear in a roadster with a sport exhaust." But, after having driving the car now miles and miles, I can't tell you how much I enjoy the system in my car- for one major reason. I can turn it up to overcome wind and road and exhaust noise, and it still sounds glorious and smooth and does not fatigue at all.
As to head units and amps, it doesn't really matter who you go with nowadays, as long as you stay with good gear. I am partial to Eclipse head units, but Pioneer and Kenwood is great too.
As to amps, again, just stay with quality. I LOVE Audison personally but again, there are plenty of good brands out there. Zapco and JL are a great value.
Unfortunately, to get the Box to sound decent requires ripping out pretty much everything that is there. You could keep the head unit if you wanted, but a replacement is cheap and a huge upgrade in terms of functionality. The factory amp is worthless, and the factory speakers are horrible as well. Once you go this far, you'll likely want a sub of sorts, so the fun continues
