Assess .php code for adoption difficiculty

One of my companies has created a new checkout software in PHP, built by former Xcart developers. Due to some very unfortunate complications, we no longer have access to our original dev team. However, we have a completed checkout software! What we need is an expert PHP developer to assess the difficulty of adopting our software to build a website with. It has minimal documentation, but has already been used to build one website. Ideally, the PHP expert we hire will give us an idea of the timeframe of adoption by a new programmer. If its decided that it is not immensely difficult to adopt, we would also be interested in paying for help turning the guidelines into documentation.