When you say healing ship, I assume you mean a ship that can heal other players, and not a tank?
The abilities you want to look at the most?
Engineering Team: It's a large flat hull heal that can be used on your self or a friendly that not be reduced or disabled. It will also allow you remove a few things that disable ship abilities. Even when you your self are disabled.
Auxiliary to Structural: A hull heal and
hull damage resistance buff that is affected by auxiliary power settings. Cycling twice as fast as a engineering team this also has the advantage of not sharing a cooldown with a engineering team.
Hazard Emitters: Function much like 'auxiliary power to structural' with two exceptions. First, It regenerates hull instead of flatly healing it. Second, it clears plasma or shield DoTs. Which is very helpful when fighting the borg. I strongly suggest this ability if no other, even with it's much longer cooldown. It also has a separate cooldown from ET and TSS.
Transfer Shield Strength: This is just plain useful to have. Aside from a flat shield heal, shield regeneration bonus that can target your self or a friendly? It will reduce the damage shields take.
Tactical Team: This ability is a must for any build. While it clears boarding parties, assorted other tactical related debuffs, and gives small bonus to weapons damage? The real reason to have this for your self or to use on a friendly? It will very quickly and automatically refocusing shield facings to where it's needed. For a healing this is a must because it means you can give your target the ability to use every shield point they have to defend them selves. Rather then the points that have on a single facing.
I neglect to touch on science teams. While it
is useful, as a heal the abilities you'll be using the most are hull heals. Simple because most players can look after their shield much faster and better then you. Which means you will be using tactical and engineering teams extensively. As all three team share a single cool down that doesn't leave much time to use all three. It's better to focus on only having or two team abilities at a time.