Mainly, armor database consists of the models the workshop made before. For example, the web site of armor workshop «Age of Craft» (http://ageofcraft.com/) offers a very large number of both ready-made armor kits and parts of armor. All the offers have already been made before, which means well-tuned production process and tests by real battles… If the customer wants something that is not included in the database, he/she shall give approximate parameters, and then manufacturers start searching for documentary evidence. If everything is OK and the evidence is found, a whole set of armor or its parts are made.
Concerning work with sources, we can say that it does not matter for a blacksmith whether they are statues, images, tombstones or armor from a museum. The most important point is to understand the details (and the more of them are available the better it is). According to some manufacturers, they managed to reconstruct armor using only an image carved on a wall. Certainly, it may be not very historical, but actually, no one can be sure about that for 100%. The blacksmith sees an image and tries to make historical armor.
There are not so many requirements concerning armor in addition to its historicity. There are certain parameters all armor kits meet, the only question is their historical correspondence. It the Middle Ages armor had a shape which meant a knight was protected from hits. Therefore, if armor is reconstructed according to the «tale quale» principle, the final product will meet the requirements of authenticity. Now, however, certain improvements and modifications of armor are used, since in olden days people were tougher and their attitude to wounds was different.