an engineer secondary that heals buildings would do good in engineer's secondary slot as it would make the rescue ranger more optional and fits right alongside engineer's other creative and building-centric secondary unlocks (the wrangler, and short circuit).

the pitfalls directly in your balance notes are not only failing to mention the range and effective healing rates, but also the idea of having an unlock that has 100 reserve ammo and little effort exerted beyond that to heal buildings, no metal or even clip size to worry about.

furthermore the engineer is trading use of the pistol for the ability to effectively infinitely heal his buildings (assuming the weapon has range, which it should if it's not going to be a worse wrench/short circuit style tazer) while the rescue ranger doesn't have healing fall off or a range to worry about, there's still the concern of clip conservation (4 shots before you have to stop immediately healing your sentry at range), and from the immediate look of your unlock it suffers from no similar downside.

the weapon's healing should have a definitive effective (and probably medium) range with healing falloff, be classified and hitscan or projectile, and most importantly consume metal to heal, and suffer from some type of buffer or cool-down that an enemy could punish.