Keyword domain

This is what I would do if I was in your situation:

Buy it, and just create a simple second website, with info on the topic, seo the site for that target keyword (hopefully you know the keyword is a well searched keyword and a converting one).

Then just simply put your link to your website all over the second website, or put your biz info in there (depending on what keyword/biz your into) that way you could end up having two websites at the top of google.

Having an exact match domain (to your keyword) helps a lot now a days.

Global targeting? Not really? Depends on the domain, what TLD it is or what you have set it to in WMT.

Could it affect your rankings, quite possibly, different URL, keyword etc if that's what you mean.

If you find domain name with your targeted keyword then I suggest you to buy it because it will help you to get quick rank on that keyword for long time as your website age.

After penguin update, Exact match keyword if use as a domain name that's not brings huge benefits.