I am looking into gettng an airbrush machine, but I really don't know anything about them. I've just heard to make sure I get one that can be plugged in. I'm not a professional and it probably won't be used a whole lot, so I don't see the need in getting an overly expensive one. Do you think this one looks like a decent one? Do you have any recommendations on one?

http://cgi.ebay.com/Dual-Action-Cake-Decorating-Airbrushing-Airbrush-Kit-/370447462956?pt=LH_DefaultDomain_0&hash=item564063b22c

No one has any advice on airbrush machines?

If you want to use one for practicing and more of hobby type work then you can get a kromojet with the small air pump and it will be fine. I started out with it and it works really well for the amount of airbrushing I do. But the downside is that the cup is small so you will have to refill your color more and the pump is smaller so less air means slower moving. But like I said, its great for beginners. Mine is the Kromo Jet 2000. I bought the whole thing which was the compressor and hose, along with the gun for about $150. If you go for the larger set then it will run your over $200
If you move into the bigger sized you can get things done much faster because of the larger cup and pump.
I bought mine from online from a cake supplies company. You can get it directly from the company and you can also buy them on ebay. I believe you can even buy them at hobby stores. I dont thing the airbrushes they sell for paint are any different, but Im not 100% positive so dont hold my word for it!
Also check out some books on airbrushing. There is one particular that is good but cant for the life of me remember the guys name. But it has a tiger on the cover of it and he has done Challenge and Ultimate Cake Challenge. He has some really awesome cakes on his site as well. If I remember or if someone else knows who Im talking about please chime in! Im so bad at remembering names!!!

Is a working pressure of 15-50 PSI for the airbrush and a working pressure of 20-30 PSI and a 55 PSI max for the compresser too much pressure?

i would say it was too much, if you will use it the majority of the time on buttercream. If you do mostly fondant then the working pressure will be fine. For the price that is not bad, the little bottles of americolor are nearly $2 a piece. It is dual action which gives you more control and its better to learn to use that first then learning on a single action and then moving to a dual action gun.