Will all contacts be delieted with an uninstall?
welcome to CiviCRM Stack Exchange. As you probably gathered from the comments from @Demerit and @petednz, if you start from scratch with a new version and "just" uninstall and remove the previous version the contacts will disappear from CiviCRM. 

It is hard to judge if they will disappear from your website without knowing your website. I do not know if for any reason users were stored in Joomla or other contact data, but you will certainly lose all the CiviCRM data.

The safest option would be to start from scratch, so get a different spot to install your new Joomla and CiviCRM. Then you could just remove your old folders and databases from your webserver once you are happy with the new situation?

Good luck!