Can I use 8.x instead of 7.x?

I want to install ELK, in an article I read 7.x or less are open source. Can I install 8.x without buying?

Yes. By default Elastic comes with a free license named "basic" which is actually super complete.

The license is super easy to understand and you have a FAQ here: FAQ on Elastic License 2.0 (ELv2) | Elastic