svt5150

I have searched for many answers but was curious how hard and if it is worth it to install a AVIC D3 in my gs400 with NAK. All I want is the AVIC D3, Carson OEM double din and really that is it. Stock sound is good with me. I have been told it is a pain and I will need atleast a new amp and sub. Otherwise they can bypass the sub but then it would sound like crap. Any help is GREATLY appreciated.... I really want to do this but without spending a grip. Thanks.