The title states it all. i need to find a wow gold class i want to play soon. I WANT to play a dps caster, but so does every man and his mother. Should i play something else that is more needed, or should i play a mage, warlock, shadow priest???
If you want something with group flexibility, go Shadow Priest. Even speced Shadow, you can heal your average non heroic 5man, it's just a bit harder since you aren't speced for it.
Play what you want.
all the classes are fun. prepare to waste days of your life finding a class. i had 66 days played on my first mage when i went back to finally finish leveling my priest, then he was my main, then i switched again, and again, eventually rerolling this mage on deathwing.

If you want dps casters that still can offer needed group utility, priests and druids (Boomkin=caster) can dps, and can spec for the much sought after healing.
You don't need 2 threads for this. play your priest. We command you.
You don't have to choose. Try a class, keep playing it if it's fun. Try another class, same deal. Also, don't ever delete a class, so you don't have to start over if you get bored with it, but want to try it again later.

You've got 10 slots to spend.
Play the class you want to play. Generally when you play a wow gold class that you enjoy, you will strive to be better at that class and therefore be more valuable to groups. You may not be the most useful but if you work at it and strive to be good I doubt you'll have trouble finding groups.