@esteller1, if I remember, your DD is CGAS, not SWAB, correct? There might be a confusion because of that. I don't think others realized it's for CGAS just reading your post. Their Cadet handbook/instructions and your Scholar handbook/instructions are similar but different. 'Hope this clarifies what others said about not needing to buy one.
SWABS will be issued new laptops, but CGAS will NOT be. You definitely might want to buy a cheap one for the year, but if your DD already has a working laptop at home, she can probably just use that until next year. No need to buy a new one just for the year. Know also that they won't be doing anything heavy-duty on laptops at prep schools. I heard you may even be able to get away without purchasing one (by using school's computer room) depending on the school. Once your DD is assigned to a prep school, the school should give you a school-specific packing list and instructions separate from the CGAS list issued by CGA. Each school will have a different list, so follow their list. Send her laptop and everything else directly to prep school later, not to CGAS summer. If you haven't already, it might be helpful to join the USCGA Scholars Program on FB to ask CGAS specific questions there. Even if the school says you can use their computers, the kids would know better how available they really are.
When my DS was sent to GMC a few years back, we probably got him the cheapest one they had at Costco. It literally broke down in a year, but he did just fine with it until his 4/c year. Going back to your MacBook question, it'll definitely work, but remember that she won't be able to use it next year after such a big investment. (Unless you want to take it from your DD for yourself next year.)