I also would head to the school of Eclipse for the AVN series of head unit. In our area Sirius is far ahead of XM, thus we sell the SIR-ECL1 sirius unit steadily. I like the new generation of ICEpower class D amplifiers, can't go wrong there. Speaker wise, consider Morel speakers, Eclipse beleive it or not, Macrom and yes, given the fact that you have a convertable and the first sound you lose at speed is top end, look to the MB Quarts. The trunk space in that you have available is far from huge. Consider a couple of 10's that can fit into either a fair size sealed or ported enclosure. The DD2510's would work for this and of course the Eclipse aluminum cone series as well. Either choice will not dissappoint.
I also happen to agree with the support the local dealer avenue. Seems that some of the suggestions on the system have come from dealers with no vested interest in the system. Too bad good dealers are so few and far between.