John, I moved your thread to the garage.
I order most of my stereo equipment to
Sonic Electronix.
An amp is always a good idea if you can find a place to put it that you like. If you're thinking of adding a footwell subwoofer, you can go with a four channel amp. With that you'll use 1 channel for left, 1 channel for right, and the last two get paired (technically it's called bridged) to run the subwoofer.
The head unit (which is the true name for what most people call the stereo) is mostly a matter of looks. If you look around, you will find opinions of all sorts regarding which head units are best. But, IMHO, the bottom line is that the signal output from all head units is
roughly the same, so buy your unit based on how it looks and features such as iPod connectivity, Bluetooth, etc. Ignore things that involve numbers because they're mostly salesman-speak (like 50Wx4 for example).