I want to be able to plug my acoustic guitar into a p.a. system (maybe good preamp too) without having to hold it in front of a microphone.

Obvious solution: buy a guitar with pre-installed pickups or get some installed into my guitar.

Do you have any suggestions as to what to look for?

My maximum budget is $300.
Does being successful truly make you a sell out? Can't you do your own original music and be successful because of it?
you could use a piezo wired straight to an output jack

or you can buy a clip on type

or you can put an electric pickup into one of those soundhole covers so its removable

the cleanest option is a piezo, and if your willing to spend money look into piezo bridges