Me being the cheep bastard i am, i was thinking of just buying a geddy lee neck as i really like them and bolting it onto my squire jazz bass and spending the rest of my money on amp/ effects...
First off, would it fit?
And second would it sound much different from my squire neck?
It probably won't do much.
Most of the sound on a Guitar acoustically comes from the Wood type and body size. Electronically comes of course from the Pickups, strings, and occasionally frets.

Changing the Neck is more of a personal preference kind of thing, the frets are really the only thing on a neck that "May" alter the sound, but only by a little bit.

Now for size, I wouldn't know too well.
TBH you'd be better served to save your money and buy a better bass. Like the Geddy Lee Jazz...
Marshall amplifiers are the truest purveyors of rock and roll known to man.

"And give a man an amplifier and a synthesizer, and he doesn't become whoever, you know. He doesn't become us."

Holy crap, check this out!
Yeahh i think im just going to buy a Geddy Lee...
Just trying to look for a cheep way out
how much better is one of them gunna play than my squire?
And how much do you think i can get hold of one for? Used or new, im not fussed...
I know brand new, the Geddy Lee goes for around 1000 canadian, hell if I know american prices :P
Those necks are so expensive secoond hand that you may as well just get the whole bass to go with it. Not to mention it comes factory with the discontinued and rare Leo Quan BAII bridge.