After paying for the purchase, you will receive an email with a serial number to activate the program. You don't need to download the program again. It is enough to call the dialog "Activate serial number ..." through the menu and enter the serial number from the letter.
You do not need to select an operating system. Serial numbers are the same for any of the supported operating systems. Just install Tofontos for your operating system and activate it with your serial number.
Oh sure. The free version is completely free and has no usage time limit as long as automatic updates are enabled.