Which way do you need to go? MTF or FTM? I know you mentioned breasts but many overweight men have breasts so I cannot tell.
I ask because the MTF route has increased risk of thrombosis and being overweight will not help. I also know that if any surgery is required, UK surgeons will not accept people over a BMI of 30 because the excess weight interferes with the surgery and the healing process.
If you are going FTM then others will have to answer as that is not my route.