"air pump" as in the emissions control part, right?
The question is why replace it. GM has a Technical Service Bulletin which disables it when it fails. If you want to do it "legally" then a dealer should do it, reflash the PCM, and add the sticker(?) and you will pay for it.
Otherwise, for probably close to the same price, you could follow the directions on the TSB [very easy; look it up online] and then have a custom tuner reflash your PCM with other performance and mileage enhancements and disable the air pump checks [which is what GM does, just without the other improvements]. Ask Anthony or Alex at http://www.pcmperformance.com for details. [They did my PCM tuning.]
Depends. If you're actually going to replace it, maybe a three, and that's providing a tube or two gives you a hard time getting disconnected.
A better, and easier option is to take it somewhere and have the airpump deleted from the pcm. I think, and someone will correct me if I'm wrong, for a '96, the pcm must be re-programmed. On a '95, the air pump relay can simply be removed and no CEL will result. Technically, the '94 is not covered by the TSB, so smog testing may be an issue, but the same methods for disabling a '95 would apply.
Keep in mind, regardless of year, the TSB is only for disabling the air pump, not removal. Some smog guys have an issue if the pump is completely removed.