Cant comment on the Alpines as I have completely forgotten what I fitted. Find some OEM door subwoofers and fit some rear speakers. Here are the door subwoofers:
Porsche 986 Boxster 996 Carrera 911 Door Panel Haes Loud Speaker Sub Woofer Left | eBay
Keep away from the Bose speakers due to the low impedance. I have a Pioneer 940BT which is a really good satnav unit. I would recommend you look at the Pioneer Appradio units rather than that Kenwood if your budget is tight. I powered the front and rear speakers off the headunit and fitted an Alpine amplifier where the Porsche amp used to be - this powers the door speakers only. You will be amazed at the difference and increased functionality.