Here is the code you might want to research if you are to be welding on armored vehicles. https://aais.ria.army.mil/aais/padds_web/W52H0908R0048/attach_exhib/att2.pdf  It will come up as this site is unsecure, but it is ok to access it.  It's the whole Tacom Cage code for armored ground combat vehicles in a PDF format.  Print it out and go through it. 
     When your contact person says that you must be certified in order to get any work, he is not really telling you much.  What code are you or your company supposed to be certified to.  When you get a purchase order from a contractor you need to show them that you have certified guys making their parts if they require your guys to even be certified.  If you tell a contractor that you abide to AWS D1.1 structural steel code(for example) and they decide to go with you because you abide to D1.1, you must show documentation proving it.  Again, only if they require it.  When you say that you have read that no matter what cert you have, you still need to test when you come into a company.  What that means is, that welders even though they may carry a cert, sometimes a company will still want them to take a test so they can be kept track of under their records.  Most of the time a welder cert is good to get your foot in the door only to take another test.  Tests cost money, so who would you want to hire to take one of your tests, a guy with no certs ever or a guy that has already got one in the test you are about to give?  Hope this kind of helps ya out.  I'm sure more people will chime in to give you even more feedback.  And when you say, we are currently looking to expand and do some work for the Deparment of Defense, I assume you mean your company.