First of I hope this goes well, I'm not an N gauge person so It's not really an appeal to me. but if I had the money I would still consider it to help out on a great idea.
As a thought for a few comments about not being able to pledge the full amount now, Could a "pre oder" option be included?
Similar to Rapido and the APT-E and using it as an example £50 now and settled by a certain date to ensure it can be done.
Worst case you get some people pull out of the preorder. I would expect these could be offered as very limited numbers for the after market. Just a thought as it might help get a few more people in.
Regardless best of luck, I'm watching this one with major interest.