Can you just buy at Best Buy and let them install it for free?
That's what I've always done. Actually, I did it at Circuit City, but I guess they're about broke and closed down now. The beauty of them was they'd put in their own adapter, so if you wanted to change after that, you just bought another adapter made for the different brand, and then plugged it right in.
But if you go to Crutchfield, they include all the wiring harnesses and everyone says it's pretty easy to wire it all up. They include specific harnesses and directions for each brand -- blue wire to blue wire, red wire to red-white striped wire, etc.
BTW, I'm like you -- no crazy speaker setups, I just wanted something that would play MP3s because my stock stereo wouldn't. Then later I changed to a Sony that plays USB drives. I'm on the road a lot and like to listen to podcasts, so I just plug the USB into my computer, load them up, then plug the drive into the car stereo. It's great -- better than when I used my MP3 player plugged into the Aux port.
Oh, and get that HD radio and let us know how it goes.
I've been curious about that, but I live too far from town and from what I hear, if the signal isn't pretty strong it will switch back and forth between the HD and analog, which can be annoying.