I think interference can depend upon where you live and what is around you. I'd buy one from somewhere with a good refund policy so you can try it and return it if it doesn't work for you. We have a Fisher Price $50 monitor- cheapest I could find and it's awesome. No interference or hassles at all.
Most definitely! And the material your house is built from, and your house layout also plays a part in it. Our Angelcare is now 4 years old and still going strong. But like meow said, with whatever you get, make sure you can take it back if it doesn't work in your house. What works for some doesn't necessarily work for others.