Fresh Installation, with all old data. Current online version CiviCRM 4.6.20. (WordPress)
We have nursed about 50 sites through such an upgrade, many of which we inherited rather than built

Firstly read the docs: docs.civicrm.org/sysadmin/en/latest/upgrade


minimise risk by doing the upgrade on a copy of the site - that way you will be able to identify the problems and come back for more specific advice once you know what issues you have hit.
identify any customised files you have on your system and assume they will break - and consider if there are better ways to manage those customisations such as in Extensions
if you have budget consider getting a Partner who can step in to do this or at least be available on an 'as required basis' to help mentor, provide emergency support etc.