Healing weapons heal your teammates only, not your enemies. There is no way to deal damage with a healing weapon.
If you try different parts in the weaponcrafting menu, the gun will show you the 'healing' stat instead of damage; you can use this to find the values you like (I'm not sure which one is the most efficient but I suspect it's beam).
In terms of choosing different types of weapons, rockets will allow you have splash healing (particularly if you use flak warheads) which can be very useful in the Blitz gamemode. Rockets can also be used to heal yourself, but the efficiency of self-healing is significantly lower than when healing another player. Beams will give you long range healing, and usually be more efficient as well, but require greater accuracy. Rifle healing seems pretty popular as well, but I've never tried it. And pulse has the bounce shot effect, which can be an asset in enclosed spaces. Rifle and Pulse weapons can also be made into shotguns. Choose the weapon type that suits your playstyle. Experiment!
Note that healing another player will also heal you, faster than self-healing with a rocket would. It's possible to overheal yourself just by healing other players