I am considering installing a suite of imacs (700Mhz) in my school and have a few questions:

Does Sibelius and Cubasis work well on these machines?
What is their 'shelf life' going to be?
If I buy controller keyboards will I need to buy any other sound hardware/software?

Any advice much appreciated

iMacs will work fine in your lab with any notation and sequencing package. The iMacs can be expected to work well for you from 3-5 years and perhaps longer. You can use USB controller keyboards using Quicktime Musical Instruments as the sound source, however I don't think you will be very satisfied with the sound quality. So you will probably want some sort of software synthesizer. One inexpensive option is the Roland Virtual Sound Canvas. You can get it for $20 US from PGMusic when you purchase their Band in a Box program which runs about $60 for educators.

If anybody else knows of inexpensive (under $100) soft synth options with GM and decent sound quality, I 'd like to hear your opinion. Reason, Unity and the stuff from Native Instruments is out of the price range for most schools.

I suppose you've looked at the emacs.

On Apple's education pages there's also a trolley with ibooks that you roll from one class to the other (difficult rolling upstairs though). I think one of the ibooks is the network boss - for you. I'm a teacher and would worry about the ibooks disappearing!!! I know I would borrow one - for work of course

Tell us exactly what you want to do with the beasties (the computers I mean).

That may be difficult - perhaps you need to know beforehand what can be done.