Ok quick update!...since my original post I found a 2009 iMac on eBay for £115...the short story is that it runs Aerodrums like a dream...

..also today I fired up GarageBand and in less than a minute it was triggering the inbuilt GB kits...no setting up internal midi/tweaking/configuration required...it just works.
Really pleased with everything and looking forward to doing some more sampling/playing/recording now!...
Cheers!