Aeriden Cleric/Undead Slayer is a good mix. The clerics typically wear robes and staves, but when they have to hunt undead obviously they can wear a bit better gear than that. Just no maiming weapons like swords and axes. Of course bludgeoning weapons wind up working better against most undead who you can't score a hit on, double damage vs skeletons. There is a bit of pacifism to an Aeridenite, but not so much so that they throw away their own lives naked and unarmed throwing themselves into the jaws of peril. I'd go with a shield and morningstar combo, and a sling. They tend to wear lots of white or silver and with gold outlines.
It can be hard levelling compared to other faiths as you are mostly pacifistic, but as has been said, you can take on the role of a party healer and try to get them to focus on undead crypts. For lower levels, Center and Vehl have nice starter crypts with lots of skeleton knuckles to use in making healing potions. Since everybody loves healing potions, you can keep making those in bulk using the enchanting skill at a local enchanting pool and in doing so become pretty good at enchanting in general. This gives you a good tradeskill to help other crafters, like jewelers who can make rings and amulets but could use a good enchanter who can enchant the gem. Alchemy and cooking are other good crafting skills for a follower of Aeriden, as you can gather most of those things peacefully. Probably not likely to get into weaponsmithing though.
You aren't totally forbidden from protecting yourself. If you go into an area and bandits attack, of course you can fend them off. The difference is the RP of it. If you're constantly attacking camps of them in an effort to kill them, that goes against the code. But if you are stuck in an area and need to get home, are trying to avoid things or even RPing that you are asking them politely to let you pass, it can be understood that you're a pacifist. We're more concerned that he is properly RPed as a non-bloodthirsty character than we want to see him die unarmed. So no using venoms, ever. In fact, fighting disease spreading and venomous vermin might be ok, as long as you're not genocidal about it. Shapeshifters and aberrations are a perversion of life almost as bad as undead.
I'd place the hierarchy of things to fight for an Aeridenite as follows.
Yes, Smite!: Undead, Shapeshifter, Aberration, Fiends, Oozes, Constructs, Summons (except druid/ranger summons & Good Aligned)
Unfortunate, but sometimes necessary: Poisonous Vermin, Plague Spreaders, Animals for Food, dangerous assassin vines, Necromancers & Corathites caught in the act of raising undead, Dangerous alien outsiders like Slaadi, a hickory tree for timber, rabid animals
Aeriden Forgive me, but I must!: Monsters curently attacking you when you didn't initiate combat, a murderer who is about to harm an innocent, a horrible monster that stalks the countryside eating children, an enemy army attacking a town that you are defending (though your role is more likely a field medic if you can manage it)
Alas! I must pray for this sin!: Animals for sport, Beasts for training, Monsters in any matter other than self defense or preserving life, Treants, Vegepygmies, bloodthirsty orcs that won't listen to reason
What happened to my spells? My holy symbol shattered!: Any sentient being that you have not given your utmost attempts to pacify diplomatically, any innocent, good aligned creatures such as fey, even the most evil creature that submits is unarmed or unaware, any of the above in a genocidal fashion (except for fiends and undead)
Mechanics wise, cleric/undead slayer benefits from gaining the Sun Domain, and as Aeriden't domains are good, healing and sun, you may want to avoid taking sun to start with since you'll get that anyhow. you can get approved for undead slayer in your initial submission if you make the bio reflect the undead slaying aspect of his training. With undead Slayer you gain new spells as you would a cleric, but do not gain increased duration or strength to them. Therefore it may be advisable to take extend spell to make your protections last on par with a full class cleric. Also avoid choosing the extra turning feat as that is already granted by Undead Slayer. Since you will be pretty turn-heavy and will want to convince quest NPCs of pacifism, charisma and persuade would be two things that come to mind when building him. Wisdom of course for spells and Strength to carry all that gear. You'll want to take at least 6 points of Lore to qualify for Undead Slayer.
Hunting grounds for intro levels include Center and Vehl's crypts. The Gloom woods a little after that, the Gloom Woods Castle, the crypts of Krandor and Storan and the Broken Halls if you have a good party and are willing to pass through a long long road to get back. From there you want to ask other players in game as I don't want to spoil the surprise.