You might want to do some bonding to the vehicle with some ground straps. Also adding a noise filter to the radio and also at the alternator will help with the noise. Take the negative and take it to a seat frame bolt or under the dash around the steering column. You can also add some ferrite chokes both at the feed point of the antenna and also one for your external speaker. Just make sure you get the correct mix or the correct ferrite chokes I should say for the frequencies being used.
As for wire, Walmart, West Marine, or any stereo shop will have good wire. For the amps you have, I would look into #2-#6AWG fine strand wire. Make sure that the negative wire is as large as the positive. And don't forget a good online fuse at the battery. Also good coax will really make a difference here. I like to use LMR240, and LMR240UF. And as far as bonding goes an easy place to start is the exhaust to frame. They sell the kits at most auto parts stores. These are just some things to think about. You may already have most of it, but it never hurts to remind. Have a Merry Christmas and a Happy New Year.
73 and God Bless.
Hope some of this info might help.