In the newer 4.3 motors like you have you can get an alt off of a fullsize and go. I did that to my extreme and it worked great for a while. I used an alt off of a fully loaded hummer when tested it was doing 220 amps which.
I don't see you having the room to do a bandpass with a 18 in the truck though. It is possible, but would be difficult to build because of various restrictions. I have helped with 160+ s10s in stock class...they can get loud and quickly. I'm not a fan of 18s in them, but to each their own. It will be loud regardless.
You will need around 6ft3 net or so depending on what 18 you eventually decide to get. AQ is a great option for the budget, but SPL makes some crazy subs with great parts.
If you need some help with a design just hit me up would happy to help you out.
I would like a single sub set up to keep it pretty simple. What would you suggest for sub and amp? And I will definitely hit you up when it's box time.