Two things you will need to consider:
1) If you want it to sound like a P-bass, you will need to install the pickup in the P-bass position. This is actually more important than choosing the right pickup (although some pickups will of course get you closer to a P-bass tone than others).
2) When you say you're building "essentially a fretless P-bass" I'm assuming you're using Fender string spacing. Make sure you get a pickup that works for the strings spacing you're gonna use. For example, TV Jones pickups look very cool and shiny but they won't work with Fender strings spacing. The E+G strings will end up outside the magnetic field and sound very weak, totally useless. MAYBE you can get away with using a TV Jones bridge pickup in the neck position but a bridge pickup is voiced for the bridge position and should be used as such. MAYBE the TV Jones blade pickups will work better with Fender string spacing than their more classic/vintage pickups... or maybe not. Whatever you do, make sure you know the pickup you get will work with you string spacing. The same applies with many other pickups too. If the pickups are designed for basses with narrower strings spacing they may not work with Fender string spacing. Some examples of instruments with narrower string spacing are Gretsch, Höfner, Gibson basses with the 3-point bridge and vintage Guild/Hagström. There are many others too but I think these are the most relevant with regards to replacement pickups. I know from personal experience that pickups from several of these basses will NOT work with Fender string spacing. Some manufacturers offer reissues of these pickups with optional string spacing though, like Curtis Novak's Bisonic/Dark Start which can be ordered both with historically accurate strings spacing, or as a Fender-compatible version with wider string spacing.
This will limit your options considerably I'm afraid. Shiny metal cover + hum-cancelling + compatible with Fender string spacing + sound close to a P-bass... Not much to choose from.