What version OS are you running currently? and what versions are you running of the software you need to use? MYOB, Office, and what else? Maybe I can help, or advise in some way... if it's not already known I am in pre-sales support/sales for Apple, specializing in the Macs, but sell everything Apple offers.
It usually isn't Apple, it's MYOB and MS. We're running Mountain Lion without issue, however; we only upgrade a year after because MYOB and Outlook typically have the biggest difficulties in clashing with other things. Outlook itself took over a year before between MS and Apple they got it right so syncing worked 99% correctly. I say 99% because Outlook 2011 still creates duplicates in contacts as it see's fit, when it feels. This means having to dedupe calendar and resync with Outlook.
MYOB software is renown for taking time before becoming completely compatible with all of the above. MYOB has to work with Excel and Outlook for most tasks.
Tell that to my Mac Mini that reboots every time I fire up the xBox to go blast some peeps in Black Ops 2 and turn my Turtle Beach XP500's on. No blue screen of death.. just a pretty parchment gray as it reboots.