Totally go for it. I love modding basses; to me, it provides a connection to the instrument that's just way, way greater than you'll have with something you bought off the rack.
In terms of what I've do:
Hipshot Cloverleaf Ultralights
Epoxy the board
Shield with copper foil
Hipshot A-Style Bridge (to be able to tweak the string spacing)
Rout out an 18V battery box
Add a preamp with active/passive switching
Pickups (any of these):
For growly (in a good way): Nordstrand NJ4SE
For burpy/smoother: Bartolini 9J#1 S/SB
For darker growly (i.e., darker than Nordstrand): DiMarzio Model J (series)
All of the pickups I listed are hum-cancelling; I like J's, but I hate hum. YMMV.
Preamp:
I'd get an active/passive push-pull setup if I went the preamp route.
Aguilar OBD3: aggressive, with HUGE deep bass
Pope Flexcore: transparent and clean
Audere Z-Mode: transparent and clean, and able to get some very cool fretless sounds with the Z-mode switch (I've heard some issues with Barts and this, however)
I wouldn't got with the tremelo bridge. I've never found them to be very useful, and I imagine that it'd make it much harder to play in tune while using it on a fretless.