I wouldn't recommend a Sport for you as you are already experiencing back pain.
Cruiser: Harley Davidson Street 750 - A very decent entry lever cruiser from HD and has enough power. But you'd have to worry about the service costs of the bike.
Tourer: Ninja 650 or ER6N - Both of these siblings have upright posture, which is very comfortable.
Triumph Speed Triple - Test rode it when I was in Mumbai, and I was blown away with it's exhaust note. Pretty powerful bike. And triumph customizes the bike according to your comfort.
And as Reddy suggested, you can test ride the Himalayan and see if you are comfortable with it. A total VFM bike and I'm sure you won't be disappointed with it.
Be very careful before you choose the right bike, opinions may vary with people. Mainly it depends on where your comfort zone lies.
Cheers. Ride Safe.