There are a few posts here about multi-speaker setups which are worth looking through to find.
Stop worrying about speaker power unless you plan to buy 1,600 watts of amplification power ($$$) across 18 channels ($$$$$$$$!!!) then the power rating of the speakers is not very significant.
I would also ensure that ALL your speakers are home run to a single amplification point. You shouldn't really be daisy chaining speakers together unless you do an ohm load consideration and factor in how it will affect power on the amplifier.
Also, you talk about 'best' and 'least expensive' in the same sentence. Serously - it's like asing what is the best, cheapest car. It doesn't really work that way. Really good speakers cost a lot more than cheap speakers, but if I were you then I would buy all my indoor speakers here:
HDMI Cable, Home Theater Accessories, HDMI Products, Cables, Adapters, Video/Audio Switch, Networking, USB, Firewire, Printer Toner, and more!
Their 8" speakers are very good, especially this in-wall model:
For only $78.59 each when QTY 50+ purchased - 8 Inches Kevlar 3-Way High Power In-Wall Speaker (Pair) - 100W Nominal, 200W Max | 8 Inches In-Wall / In-Ceiling Speakers
For outdoor speakers, I typically go to
www.outdoorspeakerdepot.com and choose something which matches my budget. You get at on of options which is nice.
There are a couple of scenarios which come at different price points.
1. Use a single amplifier and a speaker selector or impedence matching volume controls. With 10 speakers inside and another 8 outside then you probably would want at least two amps for that. Likewise, you have to deal with the stereo to mono conversion which can be handled with a converter available online.
2. Best: Get a multi-channel amplifier that is of decent quality and is 4 ohm stable and run multiple speakers at once. You need a total of 18 channels which is kind of a weird number, but you could get by with a good 12 channel amplifier.
Like this:
Sonance Sonamp 1250 MKII Powerhouse 12 CH Power Amp w High Current No Reserve | eBay
You can run one pair of speakers per channel at most. So, two groups of four outdoor speakers would take two channels. So, four channels are lost (at most) to outdoor speakers. But, it would be better to run each outdoor speaker on it's own amplifier channel. Maybe double up one set to leave you five channels left for the indoor speakers which could be grouped as a speaker pair per channel.
You would need to put some volume controls in line with everything to handle volume level control effectively.
This is still not really a 'best' scenario, but is better.
Best would be to get two of those amps. That would give you a total of 24 channels of power. You could bridge some channels to get more power to certain speakers (outdoors?) and then drive the indoor speakers on their own channels. You could use a multi-channel preamp or just an inline volume attenuator to handle the audio volume duties, or a 'smart' system which allows iPhone control of the volume in the different areas.
In my home I have about 40 speakers across 20 stereo zones. I use five 8-channel amplifiers to run them all and three 8-zone preamps to handle source selection and volume control. It is a 'best' scenario and the hardware probably runs in the $8,000+ range on the used market. Similar, for your setup (call it 7 zones) could be done for about $2,000 of hardware, but then would need to be programmed. If you are doing all the wiring, then this might still be a great way to go.
This:
Sonance Sonamp 1250 MKII Amplifier 12 Channels Bridgeable to 6 Channels | eBay
This:
Sonance Sonamp 1250 MKII Powerhouse 12 CH Power Amp w High Current No Reserve | eBay
This:
Crestron Audio Distribution Processor CNX PAD8A w Rack Ears | eBay
This:
Crestron MC2E Professional Media Controller and TPS 2000L Wall Mount Touch Panel | eBay
and this:
Crestron CNPWS 75 Watt 24 Volt Regulated Power Supply | eBay
About $1,400, and then plan about $600 to have it programmed to work with your iPhone. It will allow for up to 8 sources to be used, can even provide remote control of those sources (if available), and gives you excellent reliability.
So, for $2,000 you can really get a very good, expandable, versatile, Wi-fi controllable system. That's really cheap overall for that type of quality.
