I have really strong feelings about hyphen domains and domains other than .com.   Most of this is from running retail sites on hyphen domains and trying to explain to people by phone about the hyphen and hearing from them how they went to the wrong domain when trying to visit.  The last customers I want going to the wrong domain are the people who are typing it in!

So, after hearing this stuff for a long time I paid hideous sums to get the unhypenated domains and the guys who I bought them from told me that their typein traffic was rising every year.

But, I have those domains now and am happy... well... there is one more that I would like to get. 

They say that converts are the worst type of fanatic. 

The singular and the plural are often used with different intent.  Here is an example... Microscope - microscopes

People searching for the singluar are typically students looking to learn about the parts of the microscope, its history, or how it is used.

People searching for the plural are typically shoppers.

Which one should you purchase or use?...  If possible I would try to purchase BOTH of them because some people will not be able to remember if you are microscope.com or microscopes.com.

However I would prefer building on the plural - even if I was building an information site - because I believe that the value of the domain would be higher as a retail site rather than an info site.

www.keyword-guidez.com

You will lose traffic to keywordguidez.com, keywordguides.com, and keyword-guides.com

Not a good idea.

I would try to buy keywordguides.com.  The price might be high and it might stretch my budget... but I would splurge on it.      And only retreat if the price was insane.