Fix our hacked WP website and Update WP to the latest version

We have a static website based on the Wordpress Platform with a fully custom theme. It is a very out of date version of WP (2.8.5) and we have been reluctant to update to the latest version in case it has any detrimental effect to the theme or site as a whole (as we were told it might do by a previous web developer???)

We've just noticed that the blog posts have all disappeared and, although the posts appear blank and the website footer has disappeared, someone mentioned they saw some ads on one of the blog pages (our site is totally ad free) which leads me to think we've been hacked.

I need someone very experienced with WP, custom themes, hack attempts to do whatever it takes to...

1. Ensure all the code on the site is clean and free from viruses, malicious code or anything else.

2. Fix the blog post issue.

3. Update our site to the latest version of Wordpress.

4. Ensure our theme and all site functions run perfectly on the new WP version.

5. Test all of this on your own staging server first before moving onto our live site as we cannot afford the site to be down for any lengthly period of time.

If you're interested in the job, PM me for the URL of the site if you want to look upfront.