I run a web hosting service and have lots of experience dealing with site cleanup.
Since you didn't specify the site type, I'm quoting you cleanup on a Wordpress site, which is the most common.
Part of my task is to replace Wordpress, all themes and all plugins.
If your Wordpress install is very old, there's a chance a plugin will misbehave or that a design issue will pop up due to the updates.
Likewise, if modifications were made to theme files and not to a child theme, the modifications will be lost.
These issues will need to be addressed by a designer/developer and falls outside the scope of this bid.
I advise that you delete any unused plugins, once the sites are clean and secure.
To do the job, I will need the following information:
Domain affected.
Hosting panel login/password.
Wordpress admin login/password.
SSH admin/password (If available)