You can apply in store. If you qualify, you can purchase it then. If you borrow money then repay it, that helps your score. However, what really helps your score is a pattern of consistently paying off debt. That's why I got into the habit of making everyday purchases on a card and paying it in full each month.
If you know for sure you will have the $400 next month, and you have the $700 now, it makes sense to purchase the Mac while the promotion is still on. For that matter, charge the whole $1100 now, put the $700 in a bank account, the $400 in that bank account next month, and make the regular payments if you are sure you can be disciplined enough not to spend the money on something else. However, if you aren't sure, then that's what gets risky.
If it works like a credit card can't I just pay off the $700 immediately so my balance is $400 and pay the rest off off next month? I would like to pay it off completely by next month. Or would that have a different effect on my credit?