hello GS
I too, have strayed down the path of low D whistledom, seduced by players far better than I can ever hope to be... and what I did was simply go up to any low D player I came across and annoy them by saying 'what a beautiful sound..what on earth is it?', why that one? etc?
Good humouredly most will talk to you, but lying down at their feet saying 'we're not worthy'. is taking it a it too far...ahem
So, listen to as many as you can come across, ask very nicely if you can try the finger spacing... if you can't cover the holes -you can't play it, with the mouthpiece question, some take more puff than others, .........and they are all expensive!!.
I was watching ebay for ages until someone in Ohio was selling his, and I managed to buy a Kerry low D plastic mouthpiece monster for half the price quoted on this side of the pond.. even with shipping!
....I still have to tie weights to my RH ring finger each night to stretch my 'span' but I have heard its beautiful sound at speed courtesy of the 'maestro' at the Whitefriars session
good luck
Tim