I hate to say it but I went through a lot of custom basses before I found what worked best for me. Part of that was because my ability got better and better and I found that my needs changed over time. Years ago I started with a 36" scale six with 17mm string spacing, moved on to 35" with 19mm spacing and have now settled on a 33" scale with 16.5mm spacing. Part of that was just the fact that luthiers started using the shorter scale, and I found that it worked best for me. Anyhow, the point is you may not get it absolutely right the first time. Sometimes it's just part of the journey. If you can think of it that way than there won't be so much pressure.....but again, if what you stated is what you want than I'm sure a Brubaker would be unreal. Also, whatever you decide on, make a pact with yourself to keep it at least a year. It takes time to settle into something new. When I picked up the bass I have now I was mortified. I was disapointed right away. The sound just was not there. After some trial and era we found some pickups that worked for this bass and it sounds incredible. I was willing to go through that because it played so incredibly well....that's the other thing, if a bass just feels right than there can always be slight modifications to get it to sound more closely to what you initially envisioned.
Anyhow...step 1: Know what you want. step 2: find a luthier that you trust and like personally. step 3: put in the order and let the process happen...again, trust. Let them do their job. If they're good they'll have good communication skills while they're building your bass, but don't smother them. step 4: you have a new bass...let it settle in and give it time. As long as the instrument is resonant and plays as you want it to, the rest can be ironed out if it doesn't sound like exactly what you want out of the box.
Good luck! It can be such a rewarding experience. Enjoy it. I have to say that I feel so blessed to have a bass that was made for me, a one of a kind bass guitar that was built by someone I respect as a builder and as a person. It makes the playing experience that much more rewarding.