What alternator should I buy?

If your battery stays charged over the week, your alternator is fine and doing its job. The battery alone should light everything fine (idle or even engine off), if your connections and switches are good. That is where you need to look. Besides "low light", such bad connections can make your alternator put out too much at speed (>16 V), which can damage things. Get a cheap cigarette light voltmeter to monitor as you drive. I use one in all my modern cars.