First: Get leather armor, and wood shields. This will dramatically reduce your injury rate, and let you get champions with some armor and shield skills before you have access to forges (for me, these come several years after the fort's establishment).
Second: If possible, buy wooden weapons from the elves once you have legendary wrestlers. However, vanilla DF elves will not bring wood axes, so you will want silver from the mountainhome (you can order silver bars AND silver nuggets, as well as galena, for this).
When possible (thanks to goblinite or iron deposits), make chain mail for your military and guards, but only upgrade to plate armor when you can consistently turn out * or better armor. Start producing full body coverage.
The thing to watch for is new recruits and artifact weapons. Be sure to layer armor for the newbies, leather and chain for training, and then plate when they become champions. Artifact weapons will cause much death among the enemy and among your ranks if used in the barracks, and won't be set down. Forbid them, and use them only when you have sufficiently experienced/equipped dwarves. Else, the bits will fly and tantrum spirals will emerge.
In my experience (using the Dig Deeper (includes orcs) mod, I find legendary sword users destroy orc sieges using wooden weapons, and I've not had any problems with sparring-wounded recruits in the 15 years of military training.
(I have had to drop some constructed floors to stop the "sparring to slow death" bug, though).