replacement speakers

Hi, I'm looking to replace all the speakers in my car for better quality ones. I'm trying to come up with a parts list of things to order so I can take it to a shop and they can install it for me. I just want to make sure I have all the things I need. I came across this website...

I have a 2008 Cadillac CTS and I want to replace the factory speakers as well as install my JL 10W7 which is a 3OHM Speaker. The amp I'm thinking of buying is the DS18 H-KO2 Hooligan 2000 Watt RMS Monoblock Amplifier. Does it matter which speakers I go with? Can they be 3 or 4 OHM speakers? Or...