When servicing our air conditioner, the service man indicated that
crawl space under the house was more humid then it should be. So we
called an agency that deals with correcting the humidity problems of
crawl spaces, and was told:
1) It is more humid than it should be under the house. 
2) the dirt is not properly covered with plastic.
3) There is no siginificant mold damage (the house is 17 years old).
4) To protect from mold and miosture damage, they recommend a special
20 mil plastic covering for the dirt floor and up on all the 4 walls
covering permently the vents, a dehumidifier
to keep the moisture under 50%, and a pump that would collect the
dehumidifier moisture and any other overflow water (say from the water
heater if it breaks) and pump it outside. The price for this is
$5,800.00 for ~ 1500 sq feet of floor space.

Question: We called for advise, so we got advise. 

But, Is all this
necesaary, needed, advisable?

I could cover the floor myself with thick plastic. Is this enough?  

Can we then add a dehumidifier (I saw one at Lowes that is a free
standing unit with a tube that can go to the outside of the house for
the drainage water for $179.00 or is it possible to have a unit designed for a
crawl space progfessionally put in)?

Do we need a pump since we have never, as far as I know, had free
floating water in the the crawl space under the house. My
understanding is that the pump is a major portion of the $5,800.00
fee (but I am not sure of that).

I just don't know how to judge what is truly needed. Youre thoughts or
recommendations?

I will p[rovide any additional info you might want. We live in North
Carolina, where it is often humid.

Thank you