So if I found a 4ohm headunit rated at maybe 25W RMS per channel, I could pretty much just drop it right in and go without any major work (with the exception of maybe bypassing the amps for the rear)?

That way I could upgrade to better speakers when I can afford later. I'm not really looking for deafening loud. Just a bit louder, a bit better quality sound than stock. Nothing a good head unit and good speakers can't do.