Win7 should be fine with 2GB. According to the min requirements, 2GB are even enough for Windows 10.
The 32-bit version should be less resources-hungry and it should still work with the original product key for Win7.
XP and Vista are now unsupported so they're not the ideal choice. Besides, laptops are more prone to run into problems of software incompatibility, so it's better to stick with the OS it was born with.
If you're not going online XP could be fine, though.
As long as you're not doing memory-intensive tasks (heavy gaming, graphics, audio production, heavy multi-tasking...) I think you're fine.