Can I put a 128Gb micro sd card in a Samsung Galaxy s2?
Officially, 128GB Card is supported for the Samsung Galaxy S5 and onwards devices in Samsung releases. But technically, SanDisk’s 128GB card should work in any device that supports the MicroSDXC standard (buy/use at own risk). Check out list of supported devices. You can also verify your device at official SanDisk website here.

What if it shows an error ?

This video should help you change your mind to really go for higher storage space.
https://www.youtube.com/watch?v=JA9fp863d-o

How much of Battery life 

Galaxy S2 has only 1650 mAh Battery, but i think if you just use it as music player then it should claim for more than 1 day battery life. you can also keep it in flight mode to give you less battery drain on networks.