How about a simple
no damage
Can be destroyed after 1 hit
Sentry disabled for 5 seconds after

extra bonus - The sentry aims around erratically or just spins while under the effect of the sapper

Able to change the sentrys team ai until removed but takes one hit to remove.

Stats:

-Deal No DMG
+It changes team, causing it to shoot the team that its on.
-1 hit to remove
+Any sentries in a small distance will also be affected.
-Only 2 more sentries can be affected in the small distance.
+If sentry is wrangled, it will disable shield and not be affected by the wrangler.
-After being placed, needs 3 second cool down. It will be shortened if you pick up ammo

what about
-100% damage
-you can only place one, placing another will destroy the old one
-12 seconds cooldown
-any weapon can damage it
+you can place it anywhere, except building (its like, you can drop it like a building)
+it has a range, and it still deactivates buildings (the engineer has to find the sapper near his buildings to unsap)
+50% health

Bi-Decoder
75% sapper health
50% sapper damage
On MVM: damage's the robots
Can only use one sapper at a time
After your sapper is destroyed/used, it needs 10 seconds to recharge