Hey there.
In my experience the Gr07 will get punchier with better amplification. But it sounds great from a Galaxy S3 or any phone with a decent dac/amp.
The difference in general between higher bitrate MP3 and FLAC isn't that big. You shouldn't have any problems regarding that I'd reckon.
The MK2 is an older version, currently 3 versions exist of the GR07: "New Classic Edition", "New Bass Edition" and GR07X.
Bass Edition is just a bit bassier, while Classic version is slightly less so. Either would probably suit you. If you know that you have a specific preference for the bass range in music, get the Bass. Otherwise the Classic tends to be a bit less expensive.
A good retailer I've used is LendMeUrEars in Singapore:
http://www.lendmeurears.com/new-vsonic-gr07-classic/
http://www.lendmeurears.com/vsonic-gr07-bass-2017-edition/