You cannot transmog legacy pvp gear on your 19 unless you have reached a personal rating of 1800 or more in RBG on a main, which will allow you to purchase the replica gear off a vendor in area 52. You will be able to transmog into the replica gear.
Other than that, you will basically be able to transmog into any classic (vanilla) item fit for your armor and weapon types, with some exceptions (bugs I would guess), that allows certain items outside of that, like some random Uldum quest items for instance, etc.