How can i make my software to be purchasable with a download?

You shouldn't, instead what is better you have a serial key attached to the program. That way it lowers piracy a tad more, which is what most people do.

Basically in the installer is encrypted and ir extracts the files. Inside the installer another encrypted file is there. Now until this file is unencrypted the program does not work. So once a pay pal transfer is complete you will get the mail address, you mail them the key to unencrypt the file. Which is done through the installer it self, by entering the key [standard], or by a seperate "registration" program.

Choose a complex algorythm for the encryption.

Because the second someone buys it, whats to stop them from simply distributing? Make sure when the key is used, the best option is to link it to call back to encrypted database. So it flags the key as used, with a copy of the MAC address. So it locks that key to that particular computer. That is the ultimate security.

Otherwise an Algorythmic Serial key is the industry standard.