Custom Invoice App on Hub Spot

Need a custom invoicing app created within HubSpot that allows for a a Quote accepted in Quotient (https://www.quotientapp.com) to trigger a job/project costing automatically created in a custom-made invoice system, using Quotient Webhooks to receive a payload of data in JSON format. This could also include things like the selected items, pricing and full contact details.
This invoicing app would allow for staged payments to be created - a deposit, interim and final payments, based on % amount of the total, but creating new invoice number each time.
This Invoice app would then be connected with Xero - and each time an invoice is sent to via the new app, it would be created in Xero. Any payments made in Xero would be visible in Hub Spot.