I run a web hosting service and have lots of experience dealing with site cleanup.
I've quoted you a Wordpress site cleanup, since it's the most common cleanup job on here.
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)