Replacing 8 speaker system, choices for amps Options

[RocketScott]

So I have an 8 speaker system and I'm slowly replacing parts of it. I just put in an Alpine CDE-9845 head unit. I replaced the 8" door woofers with some Alpine 6180 subwoofers. I also have some Kicker KS60 coaxal speakers in my truck that I think I'll put in the back seat. I yanked out the two factory amps and am running everything off the HU except the subs which I now need an amp for. I'd eventually like to replace the midrange and tweeters in the doors with a component system but I haven't seen any that will fit. The smallest I've seen is around 5" IIRC. So do I get a mono amp to just run the subs for now and get another amp later to run the rest of the speakers when I upgrade or do I get one big amp now that will run it all? The head unit has two sets of RCA outs, one for the front and one for the rear/subs