Need multi vendor development

Hello
I am putting together a project using Joomla and Hikashop. I usually do most of the work myself but this is beyond my abilities. So I would like to put the project out to tender, and get a rough idea of the development cost.
Please note: I only require someone to handle the Hikashop aspect, I will be building the rest of the site myself.
The specs are as follows:
1. A multi vendor store. Each store holder must be able to handle their own store
2. Customers can purchase from multiple stores, in one transaction
3. Relevant payment for each item is to be sent directly to the individual vendors account
4. Delivery will be charged and sent to ONE additional account - so at the very least, payment will be sent to 2 accounts; the vendor and the delivery company.

Also, a question, with regard to point 3. above, do all the vendors have to use PayPal, or is it possible to send the relevant payments to each vendors chosen payment processing provider?

Development of the project will begin within the next 4 weeks.

Many thanks

Jo

Thanks Jerome
I must admit, it was the 'adaptive' payment system I was mainly concerned with. Had visions of me spending hours pulling my hair out, as is often the case when facing a new task. Perhaps I should just give it a go.
Many thanks
Jo