Your battery won't last more than a day, with Always-On Mobile on and background data turned on (which I believe is supposed to be on for Google Voice to work properly) unless you have most of your Accounts and Sync accounts unchecked for syncing, and no Mail account trying to sync, and no extra weather apps or Location apps or anything trying to sync.
My battery lasts around 3 days typically, Seidio 1750 (the stock one lasted 2-2.5 days typically), by keeping Mobile Network off unless I'm using it, only using Wi-Fi when at work (uses 1/10 the battery that mobile3g network does), and keeping my screen brightness low (0% at night, 25-40% during the day).
And, yes, Google Voice does have texting built in. Should work just fine.