~*Lil-Ms-Tweeter*~
a function of a HU is to become the source of your sound system :)
be it radio, cd, mp3, dvd etc
if you're a casual listener, can go for HU with not so extreme features/price.
if you think/believe that you are those very picky when it comes to ICE, can go for those high end ones.
the pre-out(RCA) is to connect to amps. So, it is advisable to get HU with what you need and just a little bit more. reason: if you plan to upgrade later, you might not need to change your HU if it is 'good enough'
if you're on a limited budget, just get something with the most reasonably priced and within your budget.
do you have plans/future plans for your ICE? Like how many amps want to put in, speakers?
what is the genre of music that you mostly listens to?
about the 24bit thing, if the HU is cheap, probably can go for it. If not, maybe no need because there might be a chance that you might not be able to hear the difference anyway. At least I can't.