www.edgeemu.com has the Atari ST ROMs you're looking for. Just register. It's free.
As for an emulator, I don't know what/where the best one is, but checking out this page would be a good start.
http://www.zophar.net/atarist.html
SainT would be your best bet, as it claims to support every version of Windows, but it's up to you.