Code audit
A senior software engineer should review the code base of your current Magento 1.x store and take notes about the current state of the code.
This means that the software engineer will ensure that there are no bad practices in the Magento 1.x store because that would lead to missing features in the Magento 2.x store.
Modified Magento core
An example of a bad practice in Magento 1.x is directly modifying the Magento core. Some developers do this instead of creating custom modules, which results in changes in the platform that are not visible in the code base through the custom modules. This will prevent those features from being migrated to Magento 2, since they are not visible to the developers when they review the custom modules.
If their core has been modified, then that will be the first step: moving those changes from the core to custom extensions, and restoring the original core files from Magento.
You can download any version of Magento, 1.0...