Ha, if it were me (key word) I would get the 4/3/2 channel amp with plans to use 2 channels for the front speakers and bridge the other two to mono for your sub when you get it. Then use the power from the head unit to run the rear speakers. I'm not up to date on all my decks, but if the Alpine deck you are looking at has a built in crossover all this will be a piece of cake and it will crossover the appropriate preamp lines and the line level lines to your rear speakers (most likely). If not, running the rears full range shouldn't be a problem...just turn down the gain on them. I use the crossover in my Pioneer deck for my system and it actually works quite well...better than using the crossovers in the amplifiers for some reason. Obviously you won't want to use the crossover until you have a subwoofer, but again, it's good to plan these things in advance.

But that's just how I would do it!