Yes and these are good tanks.
None I believe
Depending on where you go, some places simply have an exchange program where you trade in an empty tank with a different tank that is already prefilled with propane. Prices range from about $15 to $22 per exchange. Other places will actually fill the same tank that you bring in and charge you per gallon of propane consumed. Again, the total price will depend on the amount of propane filled in the tank, but a typical 20 lb. tank takes about 4.5 gallons of propane if completely empty. The price per gallon of propane where I am (Texas) is currently $3.29/gallon, but it fluctuates around the country much like gasoline so your cost may be significantly more somewhere else like California.
Yes at most home depots
No is for BBQ use.
Yes it is perfect for a twin tank set up on RVs.
No not a good idea to paint propane bottles. White and aluminum color are the best. Colors that absorb radiation could have disastrous results.
Most, if not all certifications on portable propane tanks is stamped on the collar of the tank, the protective steel or aluminum barrier that surrounds the valve on top of the tank. It also has the expiration date stamped on it, the date it needs to be re-certified, typically every 12 years.
Yes, the tank is empty, there are many places that fill tanks.