Most important medic traits in my opinion are:
Defender - 1st
Surgeon - 2nd
Compound Z - 3rd
Armament - 4th
Armorer - 5th
Defender is top-priority because every time you heal someone, you give them a defense, attack, and speed buff, all at the same time. The defense buff is very strong and helps keep melee mains alive when they're being hit constantly. The speed buff helps when your healing target is trying to escape. Attack only works as well if your healing target is dealing damage, otherwise, it's useless on people like caders and other medic mains.
Surgeon is great because it boosts the rate you can heal people, alongside how much you heal, effectively boosting your overall healing capability. If you want to maximize healing and minimize point loss through buying Medkit ammo, Surgeon helps with that, because the increased healing improves your profit margins, since profit for healing is based on how much you heal.
Armorer has been changed to now only give you armor as you heal, which is great for you, but of no use to your patient, making it low on the list. This trait should only be taken as a self-benefit utility.
Armament is essential for providing armor, and remains as an important medic trait even after Armorer was added. Being able to give armor while healing has its benefits by reducing damage from zombies who don't have Serrated Claws, while also increasing survivability by making most DoT effects irrelevant (poison damage is an exception, unless you have Hazmat Armor).
The problem with Armament is that your healing effectiveness is reduced, even with Surgeon to help mitigate some of the healing loss, and when zombies get/are given Serrated Claws, armor becomes irrelevant. And because this only works with medic guns and not the Medkit nor Medic Grenade, it's low on the list.
For both Armament and Armorer: Armor only serves to mitigate DoT effects from debuffs, like Burn and Poison, and direct damage. But if your healing target doesn't have Hazmat Armor, or most zombie players have Serrated Claws going into the midgame, armor becomes irrelevant, making both traits eventually irrelevant.
Compound Z is amazing for its ability to remove debuffs, and that makes it a strong utility medic trait. It beats both Armament and Armorer by a long shot on the list because it can remove every debuff inflicted by zombies in the game. This makes it essential to any medic loadout.
Note: I'm not sure if it's been changed where Compound Z's effects apply to the Rejuvenator or Medic Grenade.
However, if you're healing someone who doesn't have a debuff, Compound Z is useless. It doesn't boost your overall healing effectiveness or dispense any buffs on your patients that could help them survive longer. Nonetheless, it's the only trait that can remove debuffs, which can help in circumstances where your cade's melee mains are being impacted by more than 4 debuffs.
As for a recommended starting loadout, what I usually go for is below:
Note: This is current for today's server build.
Equipment
Medical Kit - Obviously to help heal people, but also to still be a helpful healer on objective maps when you don't get Rejuvenator.
Medic Grenade - Amazing for AoE healing, but has a long cooldown. Helpful to have when you don't get Rejuvenator on objective maps, alongside the Medkit.
Traits
Defender - For the 3-buff healing.
Surgeon - For increased healing effectiveness.
Compound Z - For removing debuffs through healing, but only with Medkit.
Armament - For being able to restore armor at reduced healing effectiveness (which is mitigated by Surgeon).
Quick and Surged - Being able to outrun a zombie is a good thing.
Fit - The extra health doesn't hurt.
Returns
Non-materialistic - There are no instances where picking up props is necessary.
Blood Thinner - Because you aren't hugging cades to shoot/attack most of the time, you won't suffer from Bleed a lot.
Palsy - Because you aren't shooting, there's no reason to have unhindered aiming ability.
Finicky - Reduced fire rate/attack speed has no effect on healing effectiveness.
Deaf - Helps to keep the micspam out, and doesn't hinder your ability to play as a medic.