Hi, I'm trying to install window 7 on my rmbp using bootcamp since some softwares that I need to use have no mac version, but currently still confused about some stuff, hopefully can get answer here

1. can I use an OEM window 7 to install? will there be any problem or should I just buy the ori one?
2. if somehow a few years after this I wanted to revert back to using OSX only on my rmbp will that be possible?
3. I'm planning to install a 64bit version, will there be any performance problem?
4. about how much storage space I should leave out for window part? my mbp only got 250GB total. Will only be using some software like 3d max,etc and maybe some light gaming on window, saving and stuff will be using external hd

tq in advance