SD billing cycle for PS

Hi,

My client is into IT service industry and they generally deals in government sector. SD module will be majorly used to bill the customer. Now we are planning the implementation cycle for SD as shown below.

Project created in PS > WBS Elements created(PS) > Value contract(SD) > Release order(SO in SD) > Invoice(SD)

Now my queries are as mentioned below

1) Is this flow from value contract to invoice correct

2) Which document type and item categories shall I use for contract, So & Invoice

3) How to integrate the billing plan

4) Pricing procedure in release orders

Thanks in advance for the support.

Regards,

HB