Motors and floaters
There have been many discussions, available in the archives, on the subject of motor size for older Macs and are a great start to get many opinions. I will risk summarizing them by saying that they mostly range from about 5hp to 9.9hp, with 7-8 being the median. The lower sizes will weigh and cost less while giving near hull-speeds in calm conditions. The larger sizes were favored with coastal conditions that include currents, wave action and strong winds. Also be aware that many lakes have restrictions for maximum size or pollution (2-cycle vs 4-stroke), so check your intended locations. As for the styrofoam blocks, they are necessary to give Macs their advertised 'positive-flotation'. While I would love to have that storage space too, I would never sail without them; I suspect that will be the prevalent advice on this site. A boat that will stay near the surface is safety and salvage smart. (That keel hangs by a pin in plastic.)