I've been using DishNetwork since they started. I've upgraded a couple of times, first to a receiver with DVR and last year to a HD dual tuner receiver, both with DVR.
On the first upgrade to DVR it wasn't something I expected to use. Boy, was I wrong. Digital Video Recording is very cool: you can pause a program (the phone rings and you want to answer it but not miss the program). Or you are watching a program and have to leave before its over, so you "back up" to the beginning, press 'record', then go back to where you were and the program is recorded up to that point and it records to the end of the program. Or you tell the machine to record every new "Earl" and then they are on your machine whenever you want to watch them.
The "dual tuner" means your receiver can be recording a show while you are watching another. Ever notice that most of the time there's nothing on worth watching but then two shows worth watching are on at the same time?
If you have caller ID on your phone, when it rings the caller ID displays on your TV screen. I can say "... it's your mother..." and continue watching while DW answers the phone and gets to listen to her mother blather on.
You can also use the dual tuner receivers to supply one TV as described above, or two different TVs. There's way more but this should be a post not an instruction manual.
High Definition is another matter. IF you have an HD TV, you might want the HD receiver. Just be aware that Dish has more HD channels but they are broadcast in "less-than-hi-def" due to bandwidth limitations. Someday it'll all be in HD but for now I'd recommend skipping the HD unless it's something you really want. Yeah, the picture quality is better but worth the extra cost? Not really IMHO.
Now, back to your question: depending on the service you want you may need to 'see' one, two or three satellites. Sometimes four. So the tree situation will certainly be a factor. You've already found sites that give you a rough idea. You probably also have an easier choice with a "bundle" from the cable co.
Phil