Update Typo3 Website from 4.7 to Current Version, incl. Custom Plugins

We have a website which needs to be updated from 4.7 to the current LTE version. There are several stock plugins in use and also some custom written ones, which still need to work on the new version. So potentially all works just out of the box by just upgrading step by step to the most current version, but potentially a lot of things break, which require minor changes in PHP source code of the plugins. We need a skilled web developer that potentially already upgraded old Typo3 to newer versions and so already knows what to tackle. Another thing is: the website got hacked, potentially BECAUSE it was running an old version of Typo3. Also part of the job would be to scan the website for potential leaks and backdoors that could have been inserted. Detials to be provided. Last: the website has to forks, meaning of: 2 copies which are very similar in all features but have different content on it. So in total that are 3 version and code identical typo3 installations. But probably the effort would just be to upgrade one and then use that code base for the others. The websites are in German and French, 2 language support. So understaning of one of the two languages apart from english would be a big pro for the job. The job should be done ASAP.