Looks like the keyboard for Surface RT is about to get a price cut in the next week which could bring surfaceRT + Keyboard into your price range. It fits your other requirements, small, light and comes with MS Office (Word, Excel, PowerPoint), 8 hours battery life and the keyboard/cover is detachable if you just want to use it as a tablet.
http://www.microsoft.com/surface/en-gb
There's a "proper" keyboard cover option as well as the touch based one.
http://www.anandtech.com/show/6385/microsoft-surface-review/4
If you're using it at a desk you can just plug in a standard USB keyboard & mouse if that suits.
Spec below but keep in mind it's WinRT rather than x86 so it won't run traditional PC apps, shouldn't be a problem though for your scenario.
OS
Windows RT; Microsoft Office Home and Student 2013 RT (Word, PowerPoint, Excel and OneNote). Works exclusively with apps available in the Windows Store.
Exterior
27.46 x 17.20 x 0.94 cm
680 grams
VaporMg casing
Dark Titanium colour
Volume and Power buttons
Storage
32GB*, 64GB
*System software uses significant storage space. Available storage is subject to change based on system software updates and apps usage. 1 GB = 1 billion bytes. See Surface.com/storage for more details.
Display
10.6" ClearType HD Display
1366x768 pixels
16:9 (widescreen)
5-point multi-touch
CPU
Quad-core NVIDIA Tegra 3
2GB RAM
Wireless
Wi-Fi (802.11a/b/g/n)
Bluetooth 4.0 technology
Battery
31.5 W-h
Cameras and A/V
Two 720p HD LifeCams, front- and rear-facing
Two microphones,
Stereo speakers
Ports
Full-size USB 2.0
microSDXC card slot
Headset jack
HD video out port
Cover port
Sensors
Ambient light sensor
Accelerometer
Gyroscope
Compass
Power Supply
24W power supply
Warranty
1-year limited hardware warranty
Apps (included)
Microsoft Office Home and Student 2013 RT1 (Word, PowerPoint, Excel, OneNote); Windows Mail and Messaging; SkyDrive; Internet Explorer 10; Bing; Xbox Music, Video and Games.