The only thing I see that might be a problem is using the DI to "check it up close" (after finding it with SI) .... reason being your transducer will be in the back end of the pontoon, & so is your "DI" & that means you'd be fishing off the back end (unless you mark the spot with a buoy and turn the pontoon around & go back to it, unless there's visible cover above the surface you can relate to).
As far as BBG .... yeah, I'd talk to him & give him the lowdown on what you're wanting and how you'll be using it. He gives discounts to Crappie.com members, so you might want to PM him, first, just to let him know your member name (& real name) & then call if needed.
As far as TM's .... I believe the highest thrust 12v is 55# --- above that they're 24v (70#-80#) You'd have to ask Brian@BBG whether any 55# TM has Spotlock or not.