We can start small (sd, hh/pedal, bd/pedal) and build on later, as to keep cost down. I would want to be able to record midi as well as audio, and mix an MP3 playing in to play along with. So, the head is as important as the pads. Mix and match works as well, obviously.
Some heads don't allow much custom mixing, etc. We don't need zillions of sets, the basics plus some fun ones, but being able to adjust the mix between sounds, and reasonable midi/audio out capabilities are a must.
This may be more about me (!) and the better the kit the better my future tracks will sound

But I am truly hoping my son gets addicted and takes this as far as he can rock.
Ken, you no doubt have opinions. And I welcome others' qualified opinions about which of the ~$500 sets are worth while. And more importantly, if you or anyone you know are selling a good set...let me know!