Question About Installing Pioneer Doube-din Av-nav Unit "avic-d2&

[darc wun]

I have a stock 03 gs300 and was thinking of buying and installing the pioneer avic-d2 in my car so that I may have nav. I have searched and read up on what is needed. So far I have gathered that it can be done, I will need the double din adapter and will lose the use of the stock sub. Is there anything else that has to be kept in mind? Will I need a special wire harness adapter will there be any issue with the car security features. What will it sound like running on the stock speaker setup? Is it possible / recommended that I also replace the stock speaker / amp. I understand 6.5 speakers fit but which one do members here recommend. Is this a relatively simple install or is it recommended that I take it to the pros.