Samoset,
I'd get a puppy before your lab/boxer passes on. The company will make him happier and it will probably extend his life. Also, there's no better teacher for a pup than another dog that knows the ropes.

I wouldn't get an aussie. They are very smart and make great working dogs but every one I've know goes full throttle all the time. They simply didn't have an off switch. I'd recommend a lab/shepherd mix.

Good luck.