The thunderbolt to firewire adapters simply work. I was worried about that too and it's been a non issue since I bought my 2013 iMac. I use an Apogee Ensemble and a UAD 2 Satellite Quad. I can even route the Ensemble through the Quad without any issues but it's better to route the interface straight into the computer. The thunderbolt adapters are firewire 800 so if you have a firewire 400 interface you should buy a firewire 400 to 800 cable. I still have my old Macbook Pro Late 2007 running 10.6.8 and Logic 8. That isn't a bus powered interface you are running so you don't need to worry about that. I'm not running any bus powered firewire stuff so I don't know the specifics of that but it's probably worth reading about if you have anything like that. As for needing new software: Logic X only runs 64 bit Audio Unit plugins. Any 32 bit plugins simply won't show up in the Audio Unit Manager. If I were you I would keep your current system intact with your current computer and start fresh on the new iMac. I upgraded to Komplete 9 for $199 on my new computer and retained Komplete 5 on the old computer and they both work fine. NI has an interesting policy about such things that says you can have Komplete installed on more than one system but you can't use them "simultaneously" and you must uninstall from one of the computers. I know that makes no sense but that's what it says but it doesn't seem to be strictly enforced. You should clone your current OS if you are going to upgrade just to be safe.