Design a web based UI for interacting with Xero and Smartsheet.

This project will necessitate familiarity with the Xero API. I am an IT service provider and I have a client who wants me to build an interface which is beyond my capability. I need a web UI for my users to approve invoices without having to use the Xero interface. The main UI will present data from Xero invoices combined with data from a Smartsheet. Users will browse through invoices select whether the selected invoice should be paid, held for further invoice or marked as in dispute. An input box for comments will need to be provided as well. If an invoice is approved for payment then the Xero status will need to be updated to "Awaiting Aproval" otherwise left as "Draft" (The MD does the final approval in Xero). The purchase order corresponding to the invoice will be matched from SmartSheet and a clickable link will open a copy of the purchase order in a new browser tab or window. So, the project will need to Read(R)/Write(W) to Xero, R from Smartsheet and R/W to a local MySQL database. The project will be housed on the company's website and users will need to log in so a user base will need to be maintained. User Names can be pulled from Xero's Payroll. A second Admin UI will need to be built where the administrator can flag which users from the Xero database are able to log in to the main UI (not all staff will be allowed to log in) and to manage credentials. The Administrator will not necessarily be on the payroll.