Welcome to the forum. This is probably a better thread for the Firearms section, maybe the mods can move it for you.
T3x stocks are generally all interchangeable, but some models have different barrel contours (e.g., Roughtech, Veil, CTR, etc.) so stocks from those models will have bigger barrel channels.
Tikka actions are all the same (i.e., there is no "long action" or "short action"). They're essentially all "long actions." You can change the chambering of any Tikka by 1) getting a new barrel, 2) getting a new bolt stop, and 3) getting a new bolt with the appropriate face (.223, standard, or magnum). Depending on what chambering your starting with, you may not need a new bolt stop and/or bolt.
Accessories and upgrades are going to depend on what you want to do with the rifle. The only "must have" Tikka upgrade for me is a stock. Nothing wrong with the factory stock, it just doesn't fit me. After market trigger springs are a common upgrade.