I would urge you to buy the Dingwall, or any other production instrument that would be a 'close enough' compromise to what you want, for two reasons:
First, unless you live within driving distance of your builder where you can go by periodically and check what this or that feels like, get together on the EXACT color you want, etc., DO NOT do this. I can almost guarantee you will be disappointed after the inevitable build time takes longer (or way longer) than what you were promised, and it generally will not match the picture you conjured in your mind over the time you've been dreaming this bass. It will also cost more. Full-tilt customs only really work for guys who've been around plenty, are in a very sympatico relationship with their builder, and realize the variations that can occur even with the very best builders out there when you leave it to them to bring your dream into the physical world . . . . from across the country, or across the world.
And as others have pointed out earlier, few builders will touch this, because they know no matter how fine an instrument they produced, there's still that divide between how they interpreted what you wanted and how that finished axe inevitably doesn't quite line up with everything you dreamt of. And the ones that will build you something from scratch, totally different than their usual menu of products, will want all the money up front. Then you're asking very talented people to step outside of their comfort zone and build something really different.
For builders, there's custom with a little 'c', and custom with a big 'C':
Custom with a little c is some variation of their regular work. Think Fender Custom Shop: Regular Fender designs with a different color, different neck profile, pickups, hardware, but still along the lines of their regular menu of instruments. Really do-able, easy variations.
Custom with a BIG C is when you want something completely off the menu. It's like telling the Gibson Custom Shop you want the greatest Stratocaster ever. Telling CF Martin you want a solid-body Explorer. You get the idea. The ones that will do this will consider you 'Proceeding at Your Own Risk' and you're gonna get their best attempt . . . . which probably won't be very satisfying.
And I certainly wouldn't do this if I were in the position of having to sell my car to do this. No Way. That sets off alarms to me that say now is not the time to do this. And after playing bass since the mid-70's, there's NO such thing as a lifetime bass: Styles, your taste, the times, everything changes over time. You will own more than a few very different basses before you're through.
Best of Luck,
JW
PS: I had a custom Alembic five-string Elan built for myself. I'd been in the Alembic community for more than a few years, knew their product range and features backwards and forwards. When the time came, I could repeat their specs and choices and features back to them, and Susan Wickersham graciously took my 'homework' and ironed-out a few loose ends for the bass I wanted the to build. It was 'custom with a little c', an Elan Five from their usual menu with some shenanigans with the pickups, wood recipe, and color. All were easily within their vast menu and knowledge base. I paid up front, received it a year later, and it was EXACTLY what I expected, because I'd done my homework, worked within their comfort zone, and was aided by Susan's almost telepathic way of sensing just what I was after.
And I promise you, I would NOT have trusted ANYONE else to get close, much less nail it, as I know the Wickershams and what Alembic does and is capable of. The thought of rank strangers building an axe for me across the country or overseas would be a non-starter for me.
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