should i erase harddrive before install?

Not unless you have some other reason to want the HD completely cleared. Installing an upgrade operating system first downloads an installation file. It then installs itself over the existing operating system. When completed the downloaded installation file is normally removed.If not, you can do so since it is no longer needed. Doing so will clear up space on the HD. Erasing a HD will remove all of your various other apps and files. Most people do this only as a last resort to solve a serious problem. Otherwise, you will then have to reinstall everything else. That is added unnecessary work.

Is this an original installation, an upgrade or a reinstallation from the Recovery Panel? Or some other?