Integrate Payment Gateway to Current Website

I require a payment gateway integrating in to my current website. There will be two companies used, PayPal and UniversalTP. The customer should be given the option to select which supplier they wish to use to process their payment. Upon making a booking, the customer should be given the option to make no deposit, £25 deposit or make full payment. If the customers booking is within 8 weeks, the customer should be directed to make full payment. Customers who make no payment or only pay a deposit should be able to make payment at a later date by being able to retrieve their remaining balance by entering their surname and booking ref number. They should then be able to make a payment amount of their choice or make full payment. Once a payment is made, the current database should be updated and payment confirmation email sent. When balance reaches £0, the booking documents should be sent to the customer. (When I manually add payments to the current system this automatically happens so unsure if anything needs changing in this area). Ideally looking for a developer for future development as well.