Welcome to the forum!
You want an ordinary car stereo. Nothing special needed. You can see some discussion of speakers and a couple of examples of others' installations here: http://www.airforums.com/forums/f450...ent-38596.html
The hum is coming from the device that converts 110v AC to 12v
DC, the Univolt. The older Univolts are notorious for overcharging batteries. They are prime candidates for replacement. The newer replacements also deliver a clean 12v
DC, sometimes the old ones didn't. There are many threads about replacing Univolts, here is a link to a Google search using the term "Univolt": Univolt - Google Search
There's plenty of reading on the topic there.