Re: Wanting to buy a modded GBA SP.
It's entirely plausible that it could be done; but slider mechanisms, especially durable, smooth, good feeling ones, are very difficult to do. The headphone thing I imagine would be no big deal.
It couldn't be made significantly smaller, and I have serious doubts that SS would be the best person to do this.
I would expect to be charged in the $200 - $400 range for this, but someone could surprise you.