Hi Lucyloo,
Yes, you should be able to use Office documents via Thinkfree:
http://member.thinkfree.com/member/goLandingPage.action When registered, you can also share documents with other peers online, without sending anything via email. The online version is paid I think, ThinkFree Office Mobile has no cost and can be downloaded via Market.
There is a HDMI video output available through a 30-pin port on the bottom of the device so you should be able to connect a monitor or a a docking/keyboard if you want, even your TV if you want to watch a video ;-)
The battery is not replaceable, strictly speaking, but you can reach it by removing the back cover carefully. Do not try this at home though because will void the warranty and just makes no sense if new. Battery should last several years.
Regarding using the GT as dedicated ereader, I would not recommend it but that's my personal opinion. For a full reading experience I think nothing replace a nice eink device. This is basically a full equipped computer (or almost) running Linux so it would be a waste of money in my opinion. Even though the battery can last a full business day (tested at work already) you will get about 9hrs of continue usage only. But mixed and on/off during the day or if you want to read full color magazines, can be used as sporadic ereader "on the go" or at bed. I do have Kindle, Aldiko and Nook app on same device, all running without an issue and without rooting or hacking Android.