I want to put an 8" subwoofer in my truck for extra bass. Help?

I own a 2005 regular cab Colorado. I have a Kenwood Excelon deck and Memphis M-Class 6 1/2" door speakers and Memphis M-Class 6 x 9" speakers in boxes behind my seats. I want some extra low end, but I listen to rock and Indie music so I want it to sound good not rattle windows and make my headlights dim, I want it to sound natural. I have no experience with this kind of stuff, but a friend offered to install the stuff for me if I would buy it and bring it to him. First, I have about 350-450 dollars to spend. Second, should I get an 8" sub or a 10" sub? I am going to save money by getting one of those 25 dollar build it yourself non-ported truck boxes off Amazon, I would like to get the rest off of Amazon. I am leaning towards the Memphis M-Class 8" sub or a JL 8" 8w7 sub, but I still don't know what amp I should buy or what amp kit or whether I should stick with a 10" over an 8" sub. Remember its a tiny truck, I only have room for a tiny non-ported box between my 6 x 9 boxes, and I don't like overpowering bass. Suggestions for subwoofer, amplifier, amp kit?