When you say speed isn't the Mainpoint about being a support priest-- you're half right. You don't need the DEX like ME priests do because most of your casts aren't 15 seconds of horrible cast time-- but you'll notice that if you compare the difference between say, 60 DEX and 80 DEX, you're going to be slower at buffing an entire party. Those small cast times add up very quickly. If you're having to cast Kyrie/Assumptio, Increase Agi, and Magnificat on a 9 person party, you don't necessarily want to have to rely on Bragi/Magic Strings to do it in a feasible amount of time.
If you plan to do PvP, remember that HP and Resists are far better than having VIT (VIT makes you a target for creators, even with the ability to Pneuma that is easily overcome by pushback). For example, my High Priest breaks 11k HP but only has 50 total VIT. If you want VIT to resist status, you're better off running interchangeable marc/ed armors with something else.
In PvE, however, VIT is important because of status resists. Players don't really have the ability as some high end monsters to just spam area-wide stuns/curses/stone curses without cast times.
INT isn't so much of import for Full Support priests. Yes, 99 INT might make your 3.5k heals (and possible 5k healbombs >.>) look really pretty, but that's a lot of stat points wasted.
You can usually go this route for PvE:
80-90 total dex (Without blessing)
60-80 total Vit
60-80 total INT (whatever is left over, again without blessing)
In PvE, your resists will mainly come from your armor DEF/MDEF, but don't forget about useful things like racial resist cards, or equipment that reduce damage (Example, Divine Cross and Spiritual Ring, or a Holy Robe). Flower Wings like Jens suggested are great for bolstering DEF/MDEF solely, but the lack of a card slot is something you might want to consider running something like a Mantle [1] / Mink Coat [1] for.
Here are some ideas for gear, both low and high end. I'll leave out cards because you can use those to personalize your own playstyle, but I'll leave ideas about useful ones.
Remember, that you don't need amazing gear to trans, or to even get 99/70. You can very easily rebirth with only store bought gear, and without any cards.
Low End Gear:
Biretta [1] - Headgear
Small Ribbon - Mid Headgear (Hey, 1 DEF is 1 DEF. And it's cute.)
Mantle[1] OR Flower Wings - Armor (Possible Lower Headgear)
Staff of Recovery / Evil Bone Wand - Weapon
Buckler [1] - Shield
Manteau [1] OR Ragamuffin Manteau - Garment
Shoes [1] OR High Quality Sandals [1] - Shoes
Clip [1]/Rosary [0], and Spiritual Ring combo - Accessories (SP recovery bonus + Healbomb bonus)
High(er) End Gear (Obviously not the extremely best gear available, but a very good step up):
Maiden's Hat [1] - Headgear
Red Glasses / Small Ribbon / Nobility Gem / - Mid Headgear
Flower Wings / Mink Coat [1] / Orlean's Robe (Trans only) [1] / ToH Wings - Armor (Possible Lower Headgear)
+7-10 Staff of Recovery / Giant Encyclopedia (The DEX bonus is pretty nice) / Divine Cross - Weapon (Keep in mind that Divine Cross is 2handed, and you lose a shield-- should only be used with Spirit Ring combo in applicable areas)
Stone Buckler [1] / Valkyrie Shield [1] / Bradium Shield [1] (Trans only) - Shield
Vali's Manteau / Wool Scarf [1] (trans Only) / +7 Muffler [1] - Garment
High Quality Sandals [1] / Tidal Shoes [1] (Trans only) - Shoes
Spiritual Ring (combo is good), Clip [1], Rosary [0]
The gear I listed is a mixture of both DEF and MDEF gear-- having them upgraded to even just +4 with some good cards can easily net you 40-50 DEF/MDEF at the same time.
As for Cards, some of the ones you may want to look at are:
Isilla
Evil Druid
Bathory
Marc
Medusa
Hodremlin
Racial Resist (Thara, Khalitzburg, Penomena)
Peco Peco
Andre Egg
Kapha
Goat
Phen / Bloody Butterfly
Raydric
Harpy
On another note, you should look into Wings, as I mentioned in the armor section. Wings to note for a FS priest are: Flower Wings, Hemerald's Elemental Wings, and ToH Wings. The Flower Wings and Hemerald's Elemental wings are questable, and can be searched up on the HeRO Wiki (The elemental wings are: Gaia Wings, Phoenix Wings, Arctic Wings, and Aerial Wings). ToH is a monthly even that requires allegiance and some knowledge about your class and how to deal with various monsters-- a great guide can be found
here (Click Me!!) They have defence and MDEF values that can rival Flower Wings, but also carry other effects, and an armor slot! Very powerful piece of equipment, but also requires preparation to get.
Other than that, I don't know. I find it's always best, and play to your own playstyle. Follow a guide and build, sure, but don't make your priest exactly like someone else's-- you won't like it. Have fun with your characters, it's
YOURS after all.