Foreword regarding items:
YOU CAN TECHNICALLY BUILD THE ABOVE BUILD EVERY SINGLE GAME, HOWEVER IF YOU WANT TO MIN MAX READ THE REST OF THIS TEXT.

You need to understand the most important Soraka concept: Soraka is, for lack of a better term, a battle healer. She has to not only be in range of her allies to heal them (which is rather short range), but also look for opportunities to Q her opponents. That leaves her much more vulnerable than other enchanters, as she participates much closer to enemy champions than them.

Our Items' main goals are: 1) scale our healing 2) increase our survivability - there is a sweetspot here. Too much tankyness and your heals are underwhelming, too much scaling of healing power and you might get oneshot at the start of the fight by random AoE effects making you useless, too.

Item Order:
1st item: ALWAYS Moonstone.
2nd Item: DEPENDS ON THE GAME. Redemption / Ardent Censer / Staff of Flowing Water / Mikaels / Locket / Shurelya etc. are ALL viable.
Redemption is the overall best item regardless of team comp. It gives the most play opportunities and has insane value overall, however it is an active item and the reality is that most people reading this guide will be in the lower elos and aren't utilizing an active item properly. If you are below Diamond, I would always recommend NOT to buy (too many) active items on Support.
Ardent is good in games where you have at least 2 auto attack champions that are your win condition (they are doing well in the game AND you can actively support them with heals in team fights)
Same goes for Staff of Flowing Water, just that in this case you look for at least 2 AP champs that are doing well in the game.
Shurelya is good with champs like Hecarim or in general comps that need to rush into the enemy team comp OR run away from them.
Mikaels is good vs stuff like Ashe arrow, however I would never recommend this item below Master.
Locket is good to gain more tankyness if there is too much unavoidable damage, e. g. Karthus R, Aurora R, Rengars, Vladimirs etc. - just generally anything that pressures / oneshots you too fast. It is the best tank item for you while still being a supportive item - but again keep in mind that it is an active item and therefore not that recommended below Diamond!


3rd item: 98 % of cases go for Dawncore - unless u need to be more tanky then u can go locket

4th item+: Any of the other mentioned enchanter items that fit the criteria

Quesitem:
Dream Maker / Celestial Opposition / Solstice Sleigh are all viable. Personally, I would recommend Celestial Opposition or Dream Maker. Celestial Opposition you should always build if you feel threatened by an assassin / dive champion. If their comp consists of a lot of ranged champs (VelKoz, Xerath, Jayce etc.) with no real dive threat onto you, you should much rather go Dream maker, as Celestial Opposition will get triggered before fights even happen and won't provide much value.