I think that the golden rule in installing an audio system applies here, especially with such a small cabin space. It largely doesn't matter the budget of the system of the typre of equipment used. It all comes down to the engineering. It certainly is easier to put speakers into the doors under the quarter window, just make sure they are very solidly mounted and as someone said before, try and eliminate the rattes beforehand. But if you are after sound quality and a decent system, then a high powered head unit and some good quality 6.5 inch speakers will suit your application. The next step up from that would be splits and then an amplifier if you wanted. But provided you took the time to kill the rattles and install the system securely, you should be happy with the above arrangement.

Hope this helps you out.