1.
Buying it in the U.S. would be cheaper, since iPad 2 isn't available in China from Apple yet and the first gen models are almost out of stock. You will need an AT&T 3G model, because the Verizon models don't have the UIM slots needed for the CDMA carrier here.
It is possible to buy from the Chinese scalpers though, cost about $50 more for the WiFi versions and $200 more for the 3G versions.
2.
China Unicom is the only carrier that provides UMTS(GSM 3G) network in China, so basically it is the Chinese counterpart of AT&T. From what I read about AT&T on these forums, China Unicom seems to have much better coverage, though the speed should be somewhat slower.
About the answer to your question, I think it depends on how far you are from the major cities. If you're just dozens of miles away from a city of eastern China, chances are you will get 3G just fine, but if you're about to get into the mideast provinces or drive through the deserts of Inner Mongolia, I'm afraid you probably won't even get a signal.
3.
I'm sure it differs from city to city, but if you buy it in Beijing, the cost would be like this:
What you need to pay: 1200yuan($180), up-front, mandatory
What you will get:
a mini-SIM card(not the micro-SIM used in the iPads, you'll have to cut it by yourself)
1200yuan worth of credit on your account
a Huawei USB dongle for your computer
The data plans are 80yuan/mo for 1GB, 150yuan/mo for 3GB, 200yuan/mo for 5GB, 300yuan/mo for 10GB, 0.1yuan/MB for extra usage and the total data caps at 15GB per month.
The only problem I can see is purchasing it probably requires local IDs, not sure if those plans are purchasable with a foreign passport, but the easiest way around would be asking one of your Chinese friends to buy it for you.
4. If it's possible, I suggest you ask your colleagues who work on the same site where you would be, ask them about whether China Unicom got good 3G coverage there, because there is a chance that your workplace isn't covered, and in that situation you will probably need to find a DSL line and a WiFI router for your surfing.
Oh and BTW, Twitter/Facebook/Youtube are all blocked in China, so are most western blog sites, porn sites and some news agencies. Wikipedia is partially blocked, the site still works but the images won't load. There are ways around the limitations and I can teach you some if you need them.