Do I need pre-outs and what voltage matters?

If you ever plan to add an external amp or subwoofer, yes. Specialist forum discussion repeatedly emphasizes 4V or higher pre-outs for cleaner signal to the amp; 2V pre-outs on entry-level units are a frequent complaint. Alpine and Pioneer mid-tier units typically advertise 4V outputs, while many sub-$100 Amazon units use 2V or unspecified pre-amps.