With the right insurance company/broker, if you have your M2 and the training course under your belt there shouldn't be a massive price difference for insurance between the 750 and 1100-1300cc cruiser market.
The biggest difference will be your ability to safely handle the bigger bike, however. Yes, a 1100+ will weigh a lot more than a 650/750 and won't handle the same, and accordingly it will take more skills to operate...so in the end starting on a smaller, lighter, more nimble bike is never a bad idea regardless of the fact it won't necessarilly save you a lot in insurance. Just sayin'.
There's tons of options in the 750 metric cruiser market so don't be afraid to consider a variety of bikes before settling on one - yes, the one that you like and feel comfortable on is the one you should buy.
As for a Harley for your first bike....lots of arguments to be made against that, including insurance costs (higher theft target, and if you finance it you'll need to carry FULL insurance instead of just liability..more $$), and also if you do drop it and damage it, well, it'll cost you a lot more to repair and potentially depreciate it's resale value.
I would recommend starting with a well used (and cheap) metric to begin with and step up from there as you get more experience, in both value and required skill.