Don't know about some M119's having air pumps and some not -- I thought they all did.
The air pump does nothing for performance. Its purpose is to supply an extra hit of oxygen to the exhaust system to allow the catalytic converter to handle the extra-rich mixture that occurs at start-up. As such the air pump only operates at start-up and usually only for a minute or two.
There are companies which rebuild MB air pumps, since the bearing is usually the thing that goes bad. Prices are MUCH better than buying new, and they come with a good warranty (3 years!). I haven't had personal experience but they seem to have a good reputation.
Here are two such (check under 'smog pumps'):