The only thing I'd be worried about with a 20D is how many clicks it's been through (i.e., shutter life), and if the sensor is still in good shape.
For lenses, sorry, no way around it, you're going to have to do your homework to learn the basics. I recommend
this website, particularly the first three sections on focal length, prime vs. zoom, and max. aperture.
Also, you're going to want to be a little clearer on "good depth of field". For landscape photographers, deep deep DoF is very very good.
I gather you want shallow, though. A wider max. aperture (smaller f-number) will do that for you.
The 50/1.8 is great if you're on a very limited budget, but it's not the only choice, or necessarily the best one. If you're working in small spaces or you want to work close to your subjects, the 50 on a crop body, like the 20D, can be a little long.