Drupal 7 - Adding an drop-down with AJAX mechanism

We have a simple Drupal 7 portal with a few custom modules. The portal is used to store the company’s products, manufacturing instructions and customer orders. The portal is not publicly accessible and no VPN will be provided. The company is an instrumentation company specializing in geotechnical applications. We need a drop-down when we edit a certain node type. The drop-down is used to select the measurement unit: Metric or Imperial. By default the drop-down will be selecting the value stored on the current user preference. The user custom field is already available, we only need to read the value and set the drop-down default to the value. When the drop-down is selected then certain form elements need to be re-render (ajax). There is already some code trying to render the elements using the user’s preference but we need to add an AJAX mechanism. The existing mechanism uses hook_form_FORM_ID_alter and hook_node_submit. The list of fields to re-render is small and the formulas are already coded. The re-render is to convert from and to metrics and imperial units. We are looking for a talented individual interested in implementing the AJAX drop-down without creating new fields in the database or to modify the existing content type. We are not looking for a client-side only solution necessarily but we are open to any suggestion. The project will be handed off through github with the database dump readily available for the developer to start. The result will be giving back using github as a pull request. More documents will be available once the candidate is selected.