i love the anwsers you get on here.
for the record
1) if they have moisture in the bank system it will show up in the tank. how bad it is depends on the moisture content.
2) leave air in the tank. just leave 500psi or a full tank. not in between. this is because if there were a fire and you were not around to get to the tanks, the overpressure plug should go before your tank explodes, or the tank will fail from heat damage before any thing else bad happens.
3) if you drain your tank empty to look inside after a vip, prepare to have it vip'ed again. if there is no air in the tank the shop will have to look inside it, so i suggest not taking the valve off yourself.
4) wipping the tank or tumbeling it will remove the rust as long as it is not extensive. if it is you will need a tumbeling expiditor to help the process along and some bottom brushes to clean the bottom of the tank. when you tumble use a joy and water solution with the media to help suspend the rust so it comes out eaiser when you drain the media.
5) after you rinse the tank when you are done tumbeling there is a flash rust preventer you can use to help stop the rust from forming. you can also use compressed air to blow out the water or buy a tank dryer stand and heat gun to help dry the tank in an effort to help with the rust.
6) if the tanks are new, and the bank system where you get your tanks filled have a low moisture content you should not have a problem with rust in the tank. since you bought them used it is hare to tell without looking in them myself how bad it is. you can whip them first and stick the end of a reg hose with the end cut off inside the tank to blow out the dust, take a look and see if that did the job. if not they should be tumbled and you will have to deal with the possibility of flash rust again
as for using a steel tank that has sat for more than a year, i would not advise. it is possiable that the oxygen content could be low enough to cause hypoxia in some cases and that would not be good. when in doubt take it to have it looked at by the pros, and dont allways heed the advise you find on here. there are plenty of knowlgeable people on here, but there are plenty who dont have a clue to some things.