had a look at your build, very nice :)
I haven't yet decided on the sub size, i've got 8's and 12's, but might go for a 10" sub as a nice medium without losing too much space to the box, i dont even know what sort of box yet, definatly sealed, but could be in the spare wheel, could be the spare wheel well, or a box like your in the side.
the system will probably end up with 3 amps (something bridged mono for the sub, large 4 channel amp bridged for the mids, and a little amp for the tweet, all run actively by my Alpine 9887r.
although, knowing me, i could go 12" sub in the boot, 8" sub/midbass in the doors, 3" mid and then tweeters, but i'd have to get a processor to do the EQ's and crossover duties! i don't even know where i'd stick all the amps for that lot!