Some thoughts on having a bass built:
-Plan your budget. Be realistic. Base cost is just that, the more you add, the more expensive it will be. You're not going to get all the bells and whistles without ponying up the money for the extras. There's price lists out there. Assuming you're looking at a Dingwall (or similar grade build) know what the ballpark cost will be. If your vision specifically has the matching headstock...do it though it's not cheap...it's in your budget, right? If not, are you willing to trade your contrast layers or one of the other specs to make that happen?
-Make your sonic plan. Have an idea what setting and/or type of tones you want. Lots of flexibility here, but no single bass can ever truly cover ALL scenarios effectively. Have a general idea of what you need to get there. Maybe you need to talk to your dealer or the shop to nail down how to get from point-A to point-B. They're pros, so they can help guide you to the right pickups, woods, layouts, electronics. Don't pick a 6 band variable Q semi-parametric preamp if a passive bass is what you want. I could have gone for a preamp a few tiers up for the Rastaburst but what I wanted to hear aligned with the Aguilar OBP-1 (2 band boost only) and the shop designed hybrid treble control (with passive roll off).
-Don't buy a custom Dingwall to replicate a custom Spector or Alembic or Wal or whatever...if that's what you want, time to look at those makers they'll make you a better Spectalembwal. Get a Dingwall because Dingwall makes the best Dingwall basses in the world. To the same extent, if you want a AB, Z, Prima...no other builder can make it with the same amount of knowledge, engineering and experience...buy the best for what they do.
-Plan the aesthetics of your build. Gonna have to balance the budget & the sonic plan here, seen? Maybe you don't have budgie and you can go all out...good for you. But plan it out ahead of time. I've done two custom builds... One was an alloutballsout job, fancy wood, complex finish, custom matching inlays, matching pickups and headstock, some stuff that hadn't been done or and we tried a few things that didn't even make it to the actual bass. Don't try to shoehorn what should be aesthetic cues from 3 basses into a single bass. It'll look like poop. My 'mission' on my super was just that...I read the brief and stuck to it. I wanted something that amounted to a 60's style P bass in a solid color. They don't all have to be bling.
-Don't order an all out custom if you're not sure you like these basses. There's plenty of production and second hand options out there to get your feet wet rather than spending lots of your cash, waiting your time in queue only to find that these designs just aren't for you. I'm still doing this. That's why I bought a cheaper used fretless 6 (parallel fret) to see if I even want to pursue another custom. Remember you spend the time & money and don't love it, you'll probably take a financial hit and there's lots of time and effort in reselling.
-If resale is important to you...build a bass that you've factored that in. Not everyone will want your pink/lavender/teal easter egg with emerald green and yellow alternating pearloid blocks. It can be done, but if you ever need to 'move it' you'll have a much harder time. When we built the Rastaburst, I knew I'd have it forever. So, none of that stuff was a worry to me. But if you're someone who's gonna take this ride then cash it in later on, be circumspect.