You buy the username, not really the download, so I would go ahead and buy it. If java is still acting up, try updating to the most recent version, and make sure you immediately go into "options" in the pause menu and put video setting to fast not fancy, and a small render distance. this should help your computer run it faster overall. minecraft is a great game, have fun!

This is a PC answer (I dont know much about Macs).

Usually if "Java stops working" it means that java is outdated, your system can't handle it, or you have too many items running (either in the background, or just running).

You can download java here - http://www.java.com/en/

If your system (computer) is outdated you usually will only need more Computer RAM

If there are too many items running you can press Crtl+Alt+Delete (Open Task Manager), and go to the "Processes tab". From here you can close out programs you are not using to save more RAM to use for MC

*WARNING* IF YOU STOP PROGRAMS IN THE PROCESSES TAB, KNOW WHAT YOU ARE CLOSING! DO NOT CLOSE ANY SYSTEM FILES, OR YOUR COMPUTER CAN BECOME DAMAGED INTERNALLY!