>I am hoping that actually the new puppy will be just as easy to train.
It's likely that your existing dog might regress for a while - it's not uncommon :)
Like Brainless I like a long age gap: young enough to enjoy a puppy but old enough for all the training to 'stick' even when being pounced upon by a shark toothed loon. Also, you want the existing dog/s to be secure enough with their place in your house/life/affections to stand being ignored for a bit whilst you do some intensive work with the new arrival.
I do think it differs a little with individual dogs. My eldest has always been a solid and sensible kind of chap and would probably have been fine at 2.5, waiting til he was 4.5 was more to do with our circumstances. On the other hand, the youngster wasn't ready at 2, better at 2.5, but at 3 and a bit I'm sure that
now is a good time for another :)