I thin the subject says it all really - I am looking to dip my toe in the world of fretless, probably better with a lined bass to start with anyway (but would consider unlined) but Jazz-proportioned necks seem to be the drill on most basses in the say sub £500 zone.
Since getting a 5 string a few years back I find switching to a Jazz width 4 string 38 mm / 1.5" nut frustratingly narrow. Amazing what a difference a few mm's seems to make...
The taper on a Jazz neck has always puzzled me a bit - perhaps there is some merit for head weight reduction but it gets pretty wide further up the neck anyway so I never really saw the appeal from a player point of view.
I think my ideal fretless would be a P-J style 4 string with a P-proportioned neck (e.g. like the Tony Franklin Fender model), but I am not aware of anything in my price range that comes close.
I have considered going 5 string fretless just to get a more substantial neck - and this my be my ignorance but I really don't understand the apparent dominance of 38mm Jazz width nuts. To make an analogy, a Stratocaster or Telecaster nut is far wider at say 44mm and most guitarists seem to get on fine with those.
I can't help wondering if Jaco had played a bass with a P neck on it things might be a bit different now
Frustratingly, nut width aside, the Squier vintage modified fretless looks like it would be a great starting point.
Maybe a self assemble from parts is the best answer but if anyone knows of a decent 4 string fretless model currently available with a more generously proportioned neck, I'd be very interested to hear about it.