I've only used electric dehydrators, but if you make one, food dehydrators essentially use constant, lower level heat and air flow to remove moisture from food and preserve it. Thus, those are the two elements that you'll need to incorporate into a homemade version.
There are many different brands, price points and features on food dehydrators. Two basic features that better food dehydrators will have are an adjustable thermostat for various heat settings and a fan with air vents to blow the removed moisture out of the dehydrator. Entry level food dehydrators typically do not have both these features.
One way to compare different brands of food dehydrators, that contain similar features, is to review the food dehydrators on a price per square foot of total drying area (total price divided by total drying area) while also considering their product reviews.
Dehydrator sizes can range from 4 square feet of drying area up to 15 square feet, so consider how often and much you'll use it. And yes, humidity can lengthen the dehydrating time.
There are numerous articles at http://food-dehydrator.com/food-dehydrator/articles.aspx
that can answer your questions in more detail. Best.