Raising PO for services(manpower)

HI

i have few questions about raising po for manpower for services for our org

a) to raise a po you need to enter the cost or price material in our case this is going to change (ie if some consultants working less or more (ot) ) in that case what is the best way to deal with the invoice or raising PO (for ex raisse a po with guess amount and when you recieve the invoice do raise cm or any thing else)

b) can i raise the PO coz we have recieved some inovices from vendors already i need to raise a PO based on invoice and how ?

c) any body has got best ex of how to deakl with pos llike charging for services (consultatns work for us)

Thank you

ash