What I would look for is a good technical editor with experience with various software technologies, and then provide whatever training was necessary to work within your workflow. For example, I have worked with several SVN workflows, and somewhat less with Git workflows, but I am confident that I could learn whatever workflow you are using because I understand basic source control principles. I have learned reStructuredText in the last few months (as has my doc team, as well as several other people on the R&D team) at work, but for anyone familiar with markup languages, learning rST will not be a problem. So I’d look for someone with experience learning new technologies quickly combined with good editing skills, as opposed to someone knowing your exact workflow and toolset.
I think the contract should be on a per-hour basis, and an experienced editor should be able to give you a ballpark estimate. If you are concerned about it getting out of hand, you can have check-ins and caps.
For the price range, I’d probably look at senior tech writer rates to come up with an estimate.