Taking payments for holiday lets?? Any suggestions.

Using version 1.3.9h with template mihluna, english version only at the moment but adding french language pack once happy with my english version.
While I'm working on the website it can be found at: template.puyrouzeau.com

I'm looking to see if anyone has any suggestions as to how this could be done. My website is for our holiday lets and I would like to be able to offer customers the ability to, if they want, pay their holiday in several installments. We often take bookings up to 10 months in advance. At the moment we ask for a deposit to secure the dates they'd like and then the whole balance is due 10 weeks before they come to stay. Ideally I was thinking of offering 3, 5 or maximum 10 installments.
We also take bookings through an agency so I don't want people to be able to just push a button and book dates until I have removed them from the agent's allocation. Is there a way they could send a request first and I give them an authorisation code to "unlock" the payment section once the dates are removed from the agent?
I was thinking of putting on as "products" each available week with a stock quantity of 1. Do you think this is the best way?

Any suggestions would be great. Thanks.

I haven't used this, so I don't know exactly what it does, but you might take a look:

http://www.zen-cart.com/index.php?main_page=product_contrib_info&products_id=588

Hi,
Thanks for the suggestion. I've had a look at the module but I wouldn't be able to use it as I use Paypal to process credit cards and the module requires you to be able to manually process the payments.