Yes, with the help of MicroSD card, you can expand the storage up to 2 TB.
Yes, Moto Z2 Play 64GB does support OTG.
There are many differences between Moto Z2 Play and Moto Z Play.
01) Moto Z2 Play is slightly thinner than Moto Z Play.
02) Moto Z2 Play weight is 145 Grams while Moto Z Play weight is 165 Grams.
Moto Z2 Play has latest Qualcomm Snapdragon 625 chipset.
Moto Z2 Play has 12MP rear camera while Moto Z Play has 16MP rear camera. But Moto Z Play had simple LED flashlight for the front camera while Moto Z2 Play has Dual LED flash with the front camera.
Moto Z2 Play comes with latest Android 7.1 Nougat OS while Z Play comes with Marshmallow 6.0.1 OS. However, Motorola has already rolled out OTA update for Z Play for Android Nougat.
Moto Z2 Play come with 3000 mAh battery while Moto Z Play comes with 3,510 mAh battery.
Yes, Moto Z2 Play does have OTG support.
Moto Z2 Play has nano sized sim card slots.