All of them will do .... you will have less flying time but will work nonetheless ....
but my personal experience, preference and what i learned from 25 years on hobby is to never save $$ on batteries and chargers.
but that just me ....
about Tuning your Bixler into a HotRod; i have one of those, check my build on the bixler thread. (2 topics in this forum)
what happens: The Bixler have so much lift within his wings that the increase in speed will force the glider to climb everytime.
I have centered all my control surfaces when i mounted my Bixler; to keep it flying level i had 1/3 of the trim for down elevator, as i increase throttle i had to keep my thumb pressing pitch down or it will climb madly ! Power is nice to have as a backup, the bixler cant be too efficient at high speeds.
Dont get me wrong, its still amazing fun to fly it but i dont recommend a power setup for the beginner.