Which dehydrator to purchase?
What are you planning to dehydrate? And for how many people? I started out with a simple round dehydrator that didn't have an adjustable heat setting, but did have a fan. A few years ago, I moved up to the Nesco line, picking up several at resale shops- I love that they have an adjustable temp and the way they recirculate the air is very different then other stack systems. I've moved on now to a used lab oven, from a local universities surplus sales- for $30. It not only dehydrates, but I can also use it to bake in or to sanitize canning jars, as it goes up 500F. Pretty good for $30. I had planned on using it as a smoker too, until I came across 2 smaller lab ovens/incubators at university surplus that were not selling at $20 each, so I offered $30 for the two together.
Given the three dehydrators you listed, I would totally cross off the Ronco off your list, as it doesn't have adjustable heat setting nor a fan to circulate the warm air. My best friend has a Ronco, and even she admits if she had to do it again, she would have went for one with a fan at least, if not a fan + adjustable temp control. For a good starter unit, the Nesco/American Harvest from Walmart is probably your best bet. It looks like WalMart has another version with a jerky extruder for making jerky from ground meat for a few bucks more, which might be helpful if you plan on making a lot of jerky.
If you are going to be doing a lot of dehydrating or jerky making, you really need to look into the Excaliber ones- http://www.excaliburdehydrator.com/De... The smallest, 4 shelf unit, is just a little over $100 at $120. The largest home unit, with 9 trays (which happen to be larger then the trays in the 4 tray unit), without a timer is just a little over $200. Excaliber is really the top of the line in dehydrators, and if money wasn't an issue, it is what I would buy for myself.
With whichever dehydrator you decide to go with, be sure to check multiple places for the best price. If you can, check out local resale shops- I've never paid more then $6 for any of mine, except my very first one I bought at a big box retail store. Ebay can also have some good bargains if you know what you are looking for....though I hope the prices will be a little higher when I finally get around to listing all the extra round dehydrators I've collected ;)
From experience I confirm the Ronco is a waste of money. You should consider only models that have both a fan and adjustable heat (Ronco has neither, and the trays are made of a cheap plastic that will start to disintegrate after a few cycles through the dishwasher). The Nesco does the job and is a best buy if you don't need large capacity. Never used the Excalibur, but if you want to step up, consider the L'equip. More expensive (similar to the Exalibur), but greater capacity and an excellent warranty.