Newer is better, so Vista would be my choice, that said, they are probably identical and it won't make any difference.
32 and 64 bit isn't the scanner, it is your PC, which is likely x64 (64 bit)
On the PC look in your C: drive, if there is a Program Files and a Program Files (x86) then you need the 64 bit driver. If there is not a Program Files (x86) you need the 32 bit.
As for disabling driver signing, no worries, it's easy, its quick, and the worst case scenario is that you will end up no farther ahead and just have to turn it back on after failing. All you would have done is wasted 10 minutes of your life that would have been better spent looking at funny pictures on the Internet. Here is one to make up for those you missed.
