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stay away from the real cheap amps like boss, pyramid, pyle, and there are many others...basically you get what you pay for. for a decent "cheap" amp expect to pay around $200.
the real cheap amps are going to be prone to serious engine noise which you don't want.

any decent coaxial speaker will work fine with just a radio and will be even better with an amp but you will be limited somewhat as to what kind of volume you'll be able to achieve.

$150 will let you get a basic pioneer cd player with rca out and detatchable face, i like pioneer for their durability and they never skip if installed securely.
just remember your audio system will only be as good as it's weakest link, if you can find a better radio/amp used you'll be better off. but i would by the door speakers brand new for the warranty since they are usually what fail first.
hope this helps you.