Door speaker install sounds like it is fine. You will probably need to run new speaker wire to the trunk if that is where you intend to put the amp.
Full range 4" for the dash is a good idea. There are no direct drop in 4" speakers though. You will need some kind f retaining ring. Be careful on depth. Not a lot of room there. DB401's fit BTW. See here:
http://www.directcircuit.com/products/Porsche-Audio-Speaker-Upgrade-Kit.html
There is a kick panel enclosure made by a company called Monney, but I have written and called them several times and gotten no response.
Take a look at this. It may interest you:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=280305308681
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