First, congratulations!! Losing 75 pounds is quite an accomplishment, and you should be very proud of yourself for having the strength and will-power to do that. Wow!
I know you must be incredibly anxious to now get rid of that excess skin that is such a visible reminder of all that you've gone through to get where you are now. Although this is probably not what you're hoping to hear, you should wait until you lose that remaining 15 pounds and stabilize your weight before moving forward with the abdominoplasty.
You're right; doing the abdominoplasty before you stabilize at your goal weight could cause you to need a second mini-abdominoplasty to get rid of additional loose skin related to losing those last 15 pounds.
Hang in there! You've got the biggest part of the battle over with and while that last 15 pounds won't be easy, you can do it. You should make an appointment with a board certified plastic surgeon who can work with you on not only reaching your weight loss goal but also in planning your abdominoplasty procedure around your weight loss plan.
Again, congratulations, and all the best as you complete your project. Good luck!!