I'd suggest looking at the devices supported by Rockbox and seeing if you like anything on that list. I've got an older Sanza Clip+ that runs rockbox fine. Never tried flac due to space restrictions. Not to mention that on a portable device you'd probably be better with something like high bitrate aac.
A stock Sansa Clip+ will play FLAC out of the box. Max officially microSD card supported is 32GB( might be able to use 64GBers but seeing posts that the player's song limit is 8,000 songs/or files) and max built in is 8GB.
IMPO this a great player and quite rugged with fully tactile control so you can operate it without looking at the screen most of the time.
I do cycling and the best way to wear it is to loop the head phone cord around my neck once and clip it to the cord and wear it like a pendant. Tie a knot in the cord to keep the weight of the buds in your ears and also if need be take up some of the slack if you have long cords.