lvl 50 whm gear?

Would say it depends on the type of end-game content you plan on doing. But let's assume you're starting from scratch, with barely anything to your name.

This should mean you have some bare essentials, such as all your AF gear, which most of it is ilvl 50 with the exception of the chest piece being ilvl 55. Your jewelry is probably lower, because jewelry unfortunately is either made or dropped in a dungeon, and the frequency of getting it from a dungeon is probably less frequent then anything else out there.

So first things first, you want to upgrade your Weapon. You can start on your relic, but you will actually be turning in that weapon instead of using it most of the time, so it's something to start on, but not be reliant on for just typical dungeon grinding for better gear. Your Grand Company Weapon is actually perfect for starting out as a new level 50. For a WHM they are all wands (1 handers) but hopefully you should be able to get a Shield for cheap, or from a dungeon (possibly).

Next I would say upgrade anything that isn't currently ilvl 50. More then likely this will be your jewelry and your belt. Again you can buy your jewelry, this isnt a problem, but if prices are a problem, I would suggest going for the lesser ones, such as the Tourmaline Jewelry, which is mostly 48 or 49, and shouldn't be too costly. As far as belts your options might be a little more limited, but a couple of runs through the Lower ILevel Dungeons at 50 should help with that, along with other gear.

Places such as The Wanderer's Palace and Amdapor Keep should each get you some Mythology as well as a bit of Soldiery to use on upgrades. Not to mention each dungeon has a nice assortment of gear suited for your level.

When you get a higher Average Item Level (around 48) you can then start to que up for Pharos Sirius, Copperbell Mines (Hard), and Haukke Manor (Hard). Depending on how well geared your groups are, these places can prove to be easy or very difficult to heal. However they will drop item level 60 gear for you.

Once you hit around 55 Item Level you can then que up for Lost City of Amdapor, Halatali (Hard), and Brayflox (Hard). Healing here can be a little overwhelming your first time, but take it slow, remember to Esuna debuffs, dodge red circles, and you should be fine. These places will drop item level 70 gear for your.

Around this time you can also start to que up for Labyrinth of the Ancients (Crystal Tower), which drops item level 80 gear and isn't too difficult to heal. It only requires item level 50 to join this raid, and thanks to the latest patch you can roll on gear without hesitation. Grab as much as you can.

Hopefully by the time you are well geared enough to take on the higher end dungeons you should be done with your Relic Weapon. Try to work on this as much as you can while still gearing yourself out. Once you have the initial weapon, which is Item Level 80, you can then focus on your gear full time. You can also save up 900 Myth and upgrade your Relic Weapon to the Zenith Weapon if you want, however you may find that your Ilvl 55 chest piece needs a big upgrade, so you may opt for the Cleric's Robe instead.

As far as priority of what to upgrade first, I would highly recommend this way:

Weapon
Chest
Legs
Head - Gloves - Feet
Belt - Necklace - Earring - Wrist - Rings

The weapon will be your biggest priority as it will increase your healing by the most amount. The added Spell Damage is directly related to your healing as a WHM, along with the added Mind, this just helps further increase your heals.

I choose the Chest next because this will be a big upgrade for you stat wise. Typically just as much as the Legs, but I've always liked to go for the chest first, just a habit of mine.

The Head, Gloves and Feet pretty much fall into the same category, they will be the next biggest thing you'll want to upgrade due to the amount of Vit and Mind they add to help you survive longer.

Belt and Jewelry come dead last, mostly because of how little they add to your overall stats. Combined they add a lot of stats, but the jewelry itself does not add vitality (unless you use materia in crafted gear), but they will add some later wanted Secondary Stats. The belt does add Vitality however, so you may prioritize it over the jewelry, but just slightly.

Just starting out at 50, I wouldn't worry about Secondary Stats too much though, such as Determination, Spell Speed, or Critical Hit Rate. These become more important when you want to start raiding the higher end content, and worry about min-maxing your gear for your own personal needs. There's always debates on these forums about which the better one is, and yada yada, but I'll just give a quick synopsis of each.

To start, secondary stats don't do much by themselves. A little crit here, some spell speed there, and you won't even notice a difference in your healing. Sure your casting time went from 2 Seconds down to 1.95 seconds, but you will never be able to tell the difference. That being said, the change comes when you stack these stats in abundance. Meaning most of your gear has one of these secondary stats on it. Such as adding over 100 determination.

As for the stats themselves you can somewhat tailor what you want out of them. I'll explain.

Determination will increase all healing done by you, this includes Cure 1, 2, 3, Medica 1, 2, and Regen Ticks. A lot of WHM's go this route because they want to squeeze every last drop of healing out of their spells as they can. However even with a fully stacked Determination set of gear, your healing spells will increase by a marginal amount. This is mostly due to the way the current gear is, and you can't add a lot of the secondary stat you want to notice that much of a difference.

Spell Speed will decrease your casting time, as well as your recast time on certain spells. Some WHMs prefer this route, as they feel getting their heals off faster can save lives, or help during those "oops" moments in fights where people just didn't move in time. Again, you will need most of your gear outfitted with Spell Speed to notice a difference, just like Determination. However, once you become more familiar with a fight, and can anticipate incoming damage, you may feel this stat isn't what you're looking for anymore.

Critical Hit Rate is typically the black sheep of secondary stats for White Mages, while it can be useful, it is also what we call "Random" and you can't rely on it. While having more Critical Hit Rate will increase your chances of getting a critical hit, you also can't expect to get a crit heal off when some one could desperately benefit from it. More often then not, you will find your Healing Crits over-healing people instead. Although it should be noted, your Regen as well as your Medica 2 regen buff, can in fact crit as well.

Piety on the other hand can be useful for starting out as a new 50. Each point of piety will increase your mana pool, which will in turn increase your mana regeneration rate (the more mana you have the higher your regen rate). Managing mana is one of the key points to being a good White Mage. You can't just spam your heals until you run out, you must time your cooldowns as well as heals and watch your mana constantly. So more mana can help you while you learn the fights and get used to healing. Think of it as training wheels almost. However, even end-game raiding some WHM's may opt to have more mana.

Whichever you choose, make sure its based off your playstyle. If you feel more comfortable with faster heals, by all means go for Spell Speed, or bigger heals go for determination. Aside from the general consensus of "Crit is bad", there really is no "bad" choice. Just your own personal preference. And again, the secondary stats add such a little amount that going one way or another won't make or break you as a White Mage. It just adds a little benefit to what you plan to do, that's all.