I have a similar situation to your clients (edit tri-level house on 1 acre with decks, patios, pool around the house where I would like wireless access) - I decided to try Tomato Firmware and use WDS to link multiple routers.
You can get refurbished linksys/cisco wireless N routers pretty cheaply on Amazon. I am using both WRT160N and E2000.
Tomato firmware can be found here »tomatousb.org/download
The FAQ describing how set up WDS here »www.polarcloud.com/tomatofaq#how···_use_wds
If on a deal I would recommend E2000 as it is easier to load the Tomato Firmware - but both routers should work.
As you'll see on my post in this forum I have some issues when trying to use WDS links to 2 additional routers - but with main main router and a second router using WDS I get great coverage and it seems stable.
My understanding is that the wired connection and wireless connections from the main router will run full speed however anything connecting wireless to the WDS station will only see 50% (of the wireless N connection) speed. This still works fine for 1080P Netflix streaming to ROKU's connecting wirelessly to WDS stations. Just to for kicks last weekend I tried 3 simultaneous HD streams - one 1080P Netfilx wired to ROKU 2, one 1080P Netflix wireless to ROKU 2 and another HD (720?) Hulu + stream wireless to another ROKU 2. Everything worked great.
I am also able to connect by my pool 100+ ft from the house. Here I don't get full speed but the link is good enough for Pandora and some video streams to iPads/iPods/iPhones.
An added benefit of the Tomato firmware is you can see the link quality and speed for all connected devices.