Moto C comes with latest Android 7.0 out of the box, so you do not require to update it immediately. However in future, if company releases update you will get it.
No. You can use only one sim card on 4G mode in this phone. You need to use 4G sim card in sim card slot one. Second slot shall use 2G sim card.
Yes, Moto C has removable battery.
There are many features which make Moto C and Moto C Plus - completely different phones.
01) Moto C has 1GB RAM while Moto C Plus has 2GB RAM.
02) Moto C has 2350 mAh battery while Moto C Plus has 4000 mAh battery.
03) Moto C has 5MP rear camera while Moto C Plus has 8MP rear camera.
But all these also makes Moto C Plus costlier than Moto C. If you are looking for normal usage - Moto C is a better option, considering the price. Both Moto C & Moto C Plus support OTG and also has front camera flash.