Buying a stereo secondhand is a great way to get a better deal. As for the amount of power you’ll need, that’ll depend on the stereo you choose.
If you want to make your listening experience a little easier with an aftermarket receiver, though, you might want closer to 200 to 300 watts RMS. If you choose to add amplified speakers, which will help cut out road noise and make your music sound clearer, you should plan on 200 to 500 watts RMS.