Basically what your trying to do is remove the apple os and replace it with another one that allows you to run a node from what your describing. I personally would go the raspberry pi route or those cheap lower powered desktop computers you find in call centers. One problem you may run into when you have a exteneral drive, If you move the cable, it can fuck everything up and you may have to restart the entire device lol. I know from first hand experience with my node.