I started "modest" (really i started "cheap") with an inexpensive sony single cd player with a removable face ($160 at Walmart and it came with two 6" 3 way speakers) and I ran it with an additional set of 6" 3 ways in the rear deck tossing all the brokedown stock speakers.
for less than $300 including wiring kit etc it sounded pretty good.
I also wanted to keep things open for future mods.
So, get a deck with CD changer controls even if you don't have a changer now, and maybe TV control if you want to add that later too. A lot of the cheap (under $250 decks) have these features.
all my stuff is Sony xPlod, and I am very happy with it in the bang for the buck for the boom. Others may sneer or look down, but hey I don't want to compete, I built my car for me.
as you can see from my sig, i kinda added to my "add on" system.