zombiehunter:
If I already have access to the compatible receiver from a different source, will that be OK?
Yes. The only thing you have to watch out for is a few IR transmitters use a different frequency. However, something like 90-95% of the IR transmitters out there use the same frequency.
I would recommend building 1-2 units first, before going full hog to build a 100 (unless time is critical, and if time is critical, and you haven't done anything with embedded processors, you are likely need to learn rather quickly). If you live in the USA, it may make sense to buy an Arduino and sensor (if they have it) at Radio Shack, so you can begin building it tomorrow (as it generally takes about a week to get things from Sparkfun unless you live near Colorado). Once you have the basic design and you know how to build it, come back and the group should probably have ideas how to make it cheaper. Sparkfun (and Adafruit) are good when you are buying a few parts, but if you need a hundred or so, there are cheaper places to get things in bulk (however, you want to make sure you know how to make the first one).
An alternative to the Sparkfun pieces, would be Digispark. You would need to get a buzzer elsewhere as I don't think they sell it, but the basic two pieces you would get are:
If you have a MicroCenter nearby, they are now stocking both Sparkfun and Digisparks.