If I previously used a Windows machine for iTunes and all of my iTunes purchases and recently bought a Mac, should I be able to see all of my purchases in iTunes on the Mac if I am logged into iTunes on the Mac?
Pro, Mac OS X (10.7), 2.3 GHz Core i7 8 GM RAM
Yes, as long as you log in with the same username. This may not necessarily be the same as your iCloud username, it depends on your past history. It's perfectly possible to have separate usernames for iTunes and iCloud, or you may have originally set up iTunes with the account which has become your iCloud account. Either way you just need to sign in with the same username as for iTunes on your Windows machine.