The concept of approval workflow

I just want to know how to do it right.

The customer wants to get a docflow with purchase orders. Some managers might approve the order and could edit some fields in the form. Any files (scans, docs) could be attached to the form. After all the filled/approved form have to be saved in a document library.

I'm not familiar with a workflow, so I think there are two ways to realize that:

1. Create a form library with a infopath template and start a workflow when a new document created. But how to get access to some fields of infopath document to modify them and how to protect some fields from editing by some managers

2. Create a workflow with variables and custom forms on each step. But how to create a filled form and any other document as a report in a document library  

Is there any recommended way to realize that I could not find any useful information in SDK and ECM Starter Kit.

Thanks in advance,