briefly my answers below:
- What is the average or prevailing rate (eruos/us) per hour?
- This is individual choice of the consultant/developer. You should email several and query them, then you can compare the prices.
- Is a down payment before work begins standard?
- I don't think it is standard but optionally a proforma invoice for a bank of hours can be issued if you agree on that
- Is payment in full before work begins standard?
- No, I don't think so
- The bug is fixed, do i have a guarantee of support free of charge if the bug returns in a certain time frame?
- Depends if you agree on that with the individual
- So the bug is fixed, we version up a month later, and the plugin no longer works again, should I pay to have it fixed again?
- Surely not but there can be rare exceptions if you hire someone (newbie developer) who does not know the code-base well and commits something really nasty (which might fix your issue anyway). Every developer contributing the code should respect the environment which means they should try and avoid causing regressions as much as possible (imho) and do not cause big changes without prior communication on the development mailing list. If they do not follow the 3Rules or commit some really bad code without willing to fix their own mistakes they will get their commit access removed and some volunteer will need to rollback their code back to a working state (that can of course include bad code contributed in good will paid by third party (you))