A: If you wipe and clean the dust and oil from metal it's not necessary. The spray paint will stick and work fine on the metal surface of the unit strut channel. In Commercial projects, I haven't see any painter to use any solvent before painting the unit strut. They just spray on it even without cleaning. Theres no issues to report. Good luck.
A: my experience says yes, clean it and an etching primer would give it a chance. if outside - get more of an automotive paint - regular rattle cans don't last.
A: yes, it might be slightly oily with the factory finish