hi people! i'm new here but i thought i'd chime in with my experience...
i went from 4 to 6 strings when my dad picked up a Brice 406HXB in a pawn shop in Arizona for $140. i was using a Vintage Stingray copy (cheap British brand, probably comparable to SX), but i had replaced the pup with a Duncan Alnico classic MM and it sounded pretty sweet! Anyway, i got the Brice and really liked it, although the neck did seem pretty huge! I didn't care for the stock pups, they were very dull, and the pre was pretty horrible, so i replaced the pups with EMGDC and CS pups and so much better! Anyway, i started rehearsing and gigging with it pretty much immediately, i mean i don't play jazz or anything so it was pretty easy to get used to, although it can still seem a bit baffling when doing any improv stuff.
anyway, i don't find any problems switching between my 4s and my 6, but am really please to have just bought a 5 from this site!
FWIW i would say, commit to buying a 5 (or 6) if you're serious about playing it. if you're gonna get bored of the novelty after 2 weeks and leave it lying in the corner, then don't bother. it really depends on what sort of player you are and also if the sound (and look!) of the bass inspires you rather than if it's hard to play or not - anything is relatively hard to play at first - if you love the sound (and look) of it you'll persist! and to that extent i would say, you can buy a cheap 5 or 6er off of ebay, but if you want a good pro, gigging sound you'll need to change the pups, and that really bumps even the cheap ones up to mid-range prices, which are at the same time used higher-end prices!!!
so i guess to conclude i would say, depending on how serious you are, see if you can get a cheap ebay 5 or 6 string bass, but used, and pay less than £100 for it, which gives you about £150 to upgrade the electronics (so £200-£300) area, or buy a used higher end bass like a Yammy TRB or German Warwick (£450-up). I, personally wouldn't bother with a mid-level Peavey Millenium or Warwick Rockbass, not that there is anything wrong with them, but they occupy the same range as the used higher-end stuff, whereas if you get a cheapy used ebay job and hate the way it feels before even getting to the sound, at least its only cost you £100.
Phew!!!!