Hi Owen Thanks very much for writing in. If you get the 12 midday bus out of Malaga airport to Marbella, you will get into Marbella arriving at the bus station on Sunday at 12:45pm on the dot. Look at this page of Costa del Sol bus timetables and then open the link "Marbella Bus Schedules" you'll find the bus route and timings for buses to Estepona under MARBELLA - ESTEPONA .
Yes, there is a different schedule on Saturday, Sundays and local or national holidays. Look under the section Sabados, Domingos y Festivos and this will give you the times for the buses over the weekend. It looks pretty good, you could catch the 13:00 or the 13:27 or the 13:54 buses to Estepona.
You can of course also take a taxi from outside Marbella bus station, there are always taxis coming and going, but if you are still feeling strong and want to continue with the fantastic savings you make with the Malaga airport to Marbella bus, then take the next bus to Estepona, the connections look good. See how you feel at this point.
I hope this helps your planning, don't hesitate to ask any more questions you might need answers for.
Best wishes from Marbella Mike Drury