I have a lvl 60 Holy Priest and he's been holy all the way. Sure it may have been a little diffacult to keep up with my mage and warlock friends but more times than not, I had to go <DND> cuz i was trying to lvl and quest but people kept askin me to run instances. Plus, when you get to 70 you actually have healing practice and you're not gonna find yourself in some raid group as the healer and not know what to do. As far as stats are concerned, get as much intallect and stamina as you can. Every point of intallect inceases mana by 15 and you need as much of that as you can get when you're healing and every point of stamina increases health by 10 and you already have a buff for that but the more the better. As of the last patch, anything that did +healing now does +spell damage but if you can find some stuff that does + a lot of spell damage, go ahead and get if. It'll come in handy.
I'm sure a shadow priest can heal instances up to a certain point but the Holy tree allows for a LOT of bonus healing making it way easier to keep up and not ending up spamming flash heal.
As far as soloing quests, as long as you keep renew and bubble shield on you as much as possible, you should be alright. Make sure you have lots of mana regenerating products (perks to having 2 mage friends) cuz i've found out that you can only kill about 2 things around the same level as you and if you're lucky you won't run outta mana and have to use a potion or wand the guy to death. Also keep SW Pain on them. It's really helpful if you do run out of mana cuz it keeps right on goin.
I'm sure a shadow priest can heal instances up to a certain point but the Holy tree allows for a LOT of bonus healing making it way easier to keep up and not ending up spamming flash heal.
As far as soloing quests, as long as you keep renew and bubble shield on you as much as possible, you should be alright. Make sure you have lots of mana regenerating products (perks to having 2 mage friends) cuz i've found out that you can only kill about 2 things around the same level as you and if you're lucky you won't run outta mana and have to use a potion or wand the guy to death. Also keep SW Pain on them. It's really helpful if you do run out of mana cuz it keeps right on goin.