Starting from SCRATCH whole home system indoor&out

My dad is putting in a stereo system and is getting advice from one person. He lives in the boonies, so I am concerned that he receive a limited amount of information. I am hoping to bounce ideas off of some of you to make sure that he ends up getting the best quality system for his money -- not just a system that is convenient for his contact.

My experience is fairly limited. I have a Denon 3803, that I found used still under warranty for $600 a few years ago and two B&W 602 speakers. I am adding on gradually as I can afford to. I suggested that I've been happy with my receiver and that he may want to get the same but wanted to get your thoughts...

Here's my dad's situation/needs:

He would like a surround sound/home theater system in the main room and to be able to power music outdoors in two separate locations (front and back yard). I have found it convenient to be able to have a movie on inside and listen to music outside at the same time and I imagine he will want the same.

Here's my dad's first message to me:
"Victor proposes to use a Denon receiver 100 watt 7.1 channel then put a booster on to power more speakers. He says to go bigger starts to cost a lot more money. Also his speakers are Canton’s – he feels the aluminum is much better than the polyprop. When you have a chance, give me your thoughts."

And his second message:
"Thanks for looking at this. It’s a Denon AVR 2807 $1000.00 AV 7002
Canton, AS80SC Canton, 2 pairs Plus X2 cantons, OES one Speaker Craft
outdoor, VBX Speaker Craft Table top Volume remote Control and a MX850
Universal Remote. I asked him to price me a DVD/VHS unit and a CD
player. Also do we need a DVD burner? I don't think so."

I don't know anything about Cantons -- have never heard of them before -- nor do I know anything about aluminum vs. polyprop. How do people feel about investing in the Denon AVR 2807 receiver? Also, how do people feel about buying used, as I did. I have seen more 3803's, like mine, available for $500 said to be in perfect shape. Perhaps I got lucky, or perhaps checking out the unit and running the serial number by corporate to make sure it is legit is a reasponsible way to buy?

Also if there other sytems/speakers that you would recommend I am interested to learn your thoughts. 7 years ago after comparing many different brands of speakers I decided to go with the B&W 602's... but I'm not sure where things stand today.

Thanks in advance for your thoughts!

-- Douglas