You can get a carvin kit with a humbucker. You just have to call Carvin and ask. Most of the woods, hardware, pups, etc. that are available on the B4 series basses (asside from finishes of course) are available on the 4 string bass kits. If you go to the Carvin forums and do a search, you can see some examples of how nice these kits can come out.
If carvin would do 1.75 nut and neck bucker only (or DIY pickup routes) I would have bought a kit ages ago.
Going Warmoth is a fine option too. Their necks are a bit chunky (just the way I like 'em) and that might turn some folks off, but I love the feel and stability of my warmoth p-bass neck.
Lastly, WD parts
http://www.wdmusic.com/ makes good parts ( I have a swamp ash jazz body that I like) and they can be ordered pre-painted or raw. They used to have pre-painted kits, but it looks like they are revamping their kit line. Still, It would probably be worth a call to see if they still offer them. Their pre-painted p or j kits for around $450 were a pretty good deal.