My service engine soon light went on and I was told by the BMW dealership a secondary are pump is needed...at a cost of $900 to repair. I would like to buy a used part myself and either try to install it myself or have local mechanic install it due to money constraints. How important is it to get this fixed right away? Can I drive it as-is for a few months without further damaging my vehicle? Any estimates of how much a local mechanic will charge?

The secondary air pump is used to reduce emissions. Having it non-functional will not impair or damage the engine. If you have emission inspections, you will not pass. Otherwise, its safe to drive until you can eventually get it fixed.