Can't help with where to train but as far as weight is concerned I am 20.5 stone and currently training, I am also over 40. Does mean I am limited to training in the more expensive C172 but I prefer that anyway as opposed to the 152.
I did the class 2 medical straight away as I thought it might be a problem and passed. The AME did comment tho that if I had been a smoker and/or drinker that may have tipped the balance. As you say, it is a good incentive to lose a few stone.