Gina,
It doesn't impact your organic listings in any way.
I also highly recommend Brad Geddes but I don't think that you need to be an excel and maths guru to use adwords effectively.
AdWords is very diverse -- you can advertise using search, display (including remarketing), & youtube -- and to great effect if done well.
The way to know if AdWords will work for you is to test it out. You may find that some terms are too competitive and certainly a lot of terms won't work well but others you didn't think about will actually convert well and you will find clicks on the cheap. Further, if you have a good ad and a relevant landing page, adwords will recognise these (reflected in their quality score) and the cost of your clicks will decrease giving you cheaper conversions.
You can measure how much your conversions from adwords are costing you if you have your analytics set up correctly. And from your description, it doesn't sound like it would be too difficult to set up.
If you make more money from the conversion than the actual cost of conversion, then it's obviously something that you should pursue.
You can also use conversion optimizer where you set a goal of how much you're willing to pay per conversion and google will optimize the campaign based on that figure.
You should continually split test -- your ads for higher CTRs and your landing pages for higher conversions.
You can target locally with adwords but depending on where you're targeting, it can be a real hit and miss