Gearing?

[Rbstr]

Get the your HQ jewelry - maybe mix tourmaline and turquoise depending on how your mana usage is. I wouldn't bother with i55 crafted stuff for the moment, just to be cheap.

Run AKs or WPs in your AF. If you're not in a bad party you'll be fine. (Though a WP "speed run" might be beyond your output for the moment)
Replace AF with strategos. Buy DL stuff to replace non-strategos stuff first (usually belt is the #1, I think). Then replace strategos stuff.

Once you've got most of your gear try to get through the ifrit/garuda maybe farm garuda for the weapon. Get relic stuff done, titan when you're >3200hp.

[Delvish]

There is a thread with a list of all available endgame gear from AF to Allagan. Pay attention to the iLV and you'll know what teir you're working with.

As a WHM, your first priority is:
MND (cure potency)
VIT (Max HP, dead healer=dead party)
and PIE (Max MP, 0MP = 0 cures)

Thankfully, all these are statically set based upon the gear ilevel. Two different i90 peices will have the same VIT, MND, and PIE (usually).
Your secondary stats are:
CRIT rate (1.5x bonus cures/dmg)
DET (cure potency, less important than MND)
Spell Speed (CURE FASTER)

These vary wildly and often times are left at 0, forcing you to choose which is better among each of the different peices. For a WHM, DET and Spell Speed are about equal in their benefit, and many a mage will go back and forth until they are blue in the face as to which is better. CRIT is not quite as useful to you as it is to a SCH, but not something to neglect. Focus on either SSp or DET, and supplement with the others.

As far as when to gear, honestly for WHM I would recommend the DL cowl. If you really don't like the look (I do not blame you) don't jump straight into the relic body. Relic peices cost precious mythology tokens that you only get max 300 every week. Start building up your nest egg or make a crafter to buy/make a Vanya Coat of Healing. It is -roughly- equivalent to Darklight, frees up a head slot, and you can customize it with materia. This is about as good as you're going to get until Allagan in Bahamut's Coil.

After that I would run The Wanderers Palace and Amdapor Keep over and over and over. Get used to it, get accustomed to your tools and endgame healing, and get gear. AK will give you many of the accessories you need, and what you don't get there you can supplement with DarkLight gear purchased through Philosophy tombstones. Personally, I would start with the ring (you can only hold one), legs, boots, then hands, and finally any accessories you do not already have from AK, or don't like and want to replace.

Use the link above to make your own decisions on what priority you want to gear up, and always always always go for your relic weapon.

[Rbstr]

You an get the Vanya Robe of healing and either the Rose Gold circlet or Vanya hat of healing instead of the Darklight cowl. With materia on it it it's better and you don't have to worry about the combined head-body if you get the Allagan healer hat drop.

It's quite a bit more total tomes in cost, though, so it would take longer. Unless you've already got quite a bit of money already.

[Taika]

Hello Haytur and congratulations on hitting 50!

As a fresh level 50, you should start with farming AK for full (or close) set of Strategos. Now that people mostly speedrun WP, AK is actually even easier to do as a WHM. I would consider WP as a waste of time for a fresh 50 as it doesn't drop the gear you need and AK gives the same amount of Philosophy for DL.

You can heal everything up to Garuda in full Strategos. For Titan, upgrade as many pieces as possible to DL and try to get Garuda weapon (and Darklight shield from WP to go with it). After beating Titan and getting your relic weapon, upgrade as much to DL and possible and weapon to +1 before starting Coil.

For crossclass abilities, Swiftcast, Eye for an Eye and Virus are a must for a healer.