I run a web hosting service and have lots of experience dealing with site cleanup.
I'm quoting an average Wordpress site cleanup. More complex sites, such as sites containing other sites and/or woocommerce will cost more for cleanup.
Part of my task is to replace Wordpress, scan(or reinstall) the active theme and reinstall 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.
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.
Cleanup time depends on site comlexity, but I should have your site back up 1-2 hours after the job is started.
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)