What size "pressure washer" will I need to remove paint from concrete or will it be best to use a "sandblaster?"

Pressure washer won't always do it for paint on concrete unless you coat the paint with a paste paint remover. Sand blasting can be rough on brick and mortar but is Ok for cement. You could look into a softer material called corn fines made from husks.

Concrete is porus, so paint and oils will soak into it. It is very hard to get it off using any method. Etching it with acid along with 4000 to 5000 PSI pressure washing ( use a flat surface cleaner attached to the pressure washer so you have a zero degree nozzle hitting the surface at close range)

The type of paint, and how long it has been on the surface makes a difference also

At times it may take washing, and acid, and washing and acid in several applications.