I'd take some time and play as many Mass Market and/or Boutique 5-String Jazz & Jazz-type Basses
before you decide to go the Warmoth route. From the type of woods you'd like
(some people prefer the natural "neutrality" of Alder while others like the mid-scoop you get from Swamp Ash) to its String-Spacing
(can range anywhere from the tight 16.5mm of the Ibanez SR line to the "full" 19mm of most Fender as well as MTD and others - basically added another string onto a 4-string Bass) and Pickups & Electronics
(from old-school Passive Pickup setup with a passive tone roll-off to Passive Pickups mated to an Active Preamp that can boost & cut various frequencies all the way to Active Pickups such as EMGs) you want to be *sure* what you like in a Bass before building one or having it built for you.
Especially with Warmoth, which the vast majority of people would agree makes & sells extremely high-quality parts, but which you'll
never get close to the money to put into it compared to a Production Bass such as a Fender, Sadowsky or Mike Lull. That's the problem with "Custom Basses"... they're custom
to you, which most people aren't.
Don't get me wrong, I think a 5-string Jazz is a *great* place to start on the Road To Fivers
(I have a Mike Lull 4 & 5, and even my Custom Spector Euro Bolt-Ons haven been modded to be half-Spector, half-Jazz), but having a Warmoth 5 built might be more than a bit premature for your first 5-string.