2. Lots of good hotels in all price ranges, from $3 guest house to the Raffels.
3. I've always hired a moto driver for the duration of my trip. Not a guide, I've visited independently. If you are in a group it may be good to hire a guide and driver, would be more confortable anyway. You can inquire at your hotel for recommendations. Expect about $20 per day for the Toyota + fee for guide. Motos run about $8 and up per day.
4. Don't know about any scams at Angkor Wat per se however there are lots of people that want your money, not typically a problem though. If you travel overland from Bangkok through Poipet arranging a Taxi to Siem Reap can sometimes (especially the first time) be a hassle. The Tales of Asia link below has just about the best info on the border situation on the net.
Tips: The sunrise at Angkor Wat is nice but you need to get there before sunrise (can be hard for me), also at Sras Srang (kind of a big pond) is good. Lots of good places to eat and fairly inexpensive however the main tourist places are typically higher than the equivalent in Thailand. The typical tourist "tour" of Tol Sap lake is good, espeically if you only have a few days.
Have fun!
You may find the info in these links helpful.
Tales of Asia - Home
Cambodia travel guides: Siem Reap, Angkor, Phnom Penh, Sihanoukville - hotels, resturants, maps, airlines and more