Couple things:
Heal is flat out superfluous.
-The cooldown is too long on it to be worth getting. A small amount of cd reduction far outweighs getting heal.
-Flash will be far more useful as far as survivability and getting around the map to support your teammates throughout the entire game.
- Clarity would be far more useful if the reason you can't heal is your oom. Also, clarity increases your support capabilities, since you have no way without it to grant mana to your teammates.
Boots of Swiftness:
- I believe the cd reduction boots are far more useful, that extra speed doesn't really do a lot for Sona (especially if you have teleport), since she should always be with at least one of her teammates anyways. It's just going to allow her to outrun them, and Sona shouldn't be in front.
Health Pots:
- Unnecessary. If you are really being hit that hard, your gold is better spent on building a Shurelya's (mana regen+health regen+health bonus). Plus, you have a heal. While yes, it's not cost effective at early levels, by the time its level 3, its dependable and cost effective.

Anyways, just some thoughts. :)

Eruner, see Summoner's Spell Chapter (updated) as my answer.
You have a quite good build with Janna... but what trying to say here in this build is a HEALER! :)

why Heal and Teleport instead of Flash and ClairVoyance?
You already have a healing spell, havent you?