Very wise. 
Do a search for “Chinese Lumens”.
The graphs here: Cree, Luminus, Seoul Semiconductor, and Nichia LED comparison
show some output curves from different LEDs.
The XM-L2 can put out about 1200 lumen at 5 amps.
Under 1000 lumen at 3 amps is more realistic, if the light is very well built.
Some good quality cells are the 3400mAh Panasonic protected 18650s.
18650s are the size (1.8x6.50cm) used in laptop batteries.
You know the difference between protected and not protected ?