Honestly if I was building a sq system with that much space constraints I would use one of Memphis or JL's flat subs and some micro zed amps. Nice 4 channel for a 2 way front stage and a small mono amp for the sub or you could get a 5 channel and do the whole system with one amp but personally I like the looks of 2 amps over one if their gonna be seen. And now a days with all the great oem integration not much use in buying a head unit unless you just want more features. As for mids highs I have always been partial to morel, JL, Infinity kappa's, and focal. Any one of those in a proper install should sound pristine if set up properly.