I bought the 6s plus on launch day at the Ginza store, SIM unlocked, and then signed up with an MVNO. The phone model is A1687. You can google the various models to confirm country band coverage.
Regarding an MVNO, I recommend the BIC Camera SIM which is provided by a company called IIJ. Data plans are cheap and coverage is good. It uses Docomo's network. You will need to sign up for a 1 year contract and pay a termination fee if you leave early but it wasn't outrageous from what I remember.
Regarding the big carriers, yes they subsidize the phone price but charge a lot more for their monthly data plan. If you do the math, it's quite a bit cheaper over 2 years to go the route I did.
Hope that helps.