Best thing to do is start off with a class 2 and go from there. A high BMI in itself is not a reason to be declared unfit. You will, throughout your career , be subjected to more frequent testing to ensure your parameters are within limits, particularly your sugar levels. This will coincide with frequent ECGs, lipid profiles and other tests associated with degrading health generally associated with DM.
Also, a Class 1 is basically a prerequisite to your exercising the privileges of your license. You can fly privately, with a class 2 but not for reward or profit on any license. It is not an airline requirement but a regulatory one.
I would suggest a consult with an aviation medicine specialist who will be better equipped in guiding you through this integral requirement of being a pilot.
Good luck.