Hate driving on A7 so prefer to get around on the bus.

Best thing to do is look at Google street view and familiarise yourself with the stretch of road near to the old town. I've gone through Marbella several times by bus from Estepona to the bus station, and there are frequent stops.  It has fixed stops and request stops, so just watch what other people do and get ready to hit the button when you're in your zone.

Can't remember the price of a ticket but offhand it's peanuts: a couple of euros, no more than 3, assuming your journey isn't further than from Estepona.

We would be travelling from El Rosario to Old Town Marbella and back. Thank you.