@Melderon Hard to see. I did pick the original one myself.

I have updated sheet with some items that are now interesting due to the item update and phased item rollout. An item like Star of Mystaria was simply a worse Diana's Pearl Necklace before, so it wasn't there originally.

The cloak choice needs to be updated. Spritecaster Cape, from BRD, should be the pick here, not Shroud of the Exile which forces you to pass on one of the best healing rings in the game.

Also the offhand needs updating. Spirit of Aquementas (20 dmg), from Linken quest line, should be the pick here.

And on your site's list, Banthok Sash is listed as 12 spirit when it should be 12 spell damage.

This is a shadow priest gear simulator written by Hrishi/Deathlace (Nostalrius) and expanded by myself, Niche (Lightbringer). Make a copy for your own editing.

https://docs.google.com/spreadsheets/d/1F1IVqSt1MEJ9yjZ9rn1aKGIWNkJiA_CNmn6fLUpakb4/edit#gid=600475609

Being a Shadow Priest in Classic:

The viability of shadow will depend on how Blizzard implements the debuff limit. In the original game, there was an 8 debuff limit until patch 1.7 (Zul'Gurub) expanded that to 16. With 8 debuffs, a shadow priest is impossible to play effectively in a raid.

If Blizzard goes with 16 debuffs from the beginning, shadow will be viable right from the start, with your raid using good debuff discipline. There is a hard limit of 1 shadow priest per raid. In most cases you will likely be last on premium caster gear because of what I'll talk about next, but as you'll see using the simulator, priest dps doesn't scale all that well with gear anyway.

[Edit - Blizzard Classic team has stated the game will launch with 16 debuffs]

Most raid bosses are fairly short fights, and shadow's mana weakness can be propped up by potions, elixirs and regen gear choices. This changes wholesale in Naxxramas where most progression fights will make any reasonable shadow dps rotation impossible to maintain, and access to debuffs will be restricted as raid damage increases and thus defensive debuffs will be required.

It is at that time I recommend switching to healing, solving both the issue of being unable to dps effectively, and your guild's sudden need for more healing in the raid. As you progress through the game, try to pick up all 8 pieces of Tier 2 and quality gear in other slots, as many priests will look past it for stronger itemization on a per-item basis. The 8/8 T2 set bonus is strong and efficient, making it a perfect set to transition to healing without dramatically affecting other priests' gear progress while you are shadow.

There are 3 typical healing specs you can choose from, a pure healing spec and two hybrid utility specs. These are Power Infusion spec, which boosts a caster's dps temporarily, and Holy-Weaving spec, using Shadow Weaving from the Shadow tree to apply that debuff to boost warlock dps.

Taken as a whole, starting as dps while healing demand is low, transitioning to healing when healing demand is high, it is plain to see priest is an interesting, flexible class. This plan helps a guild retain the same players for the entire content run, rather than replacing a dps member with a new healer recruit for the last raid.