Twin wire boat

Highlander

with 4K  in your  pocket, maybe go for a  RS800 -  same designer as the 5000, but more refined and  less powered up but a good boat to sail.

Also what do you want to do with the boat.... go for a burn or race competitively (against what kind of  competition)...

If  you're strong enough - go for a 5000 but take note. I  am a huge fan of the 5000 (prefering it to say the 49er), but it is a powerful boat. It handles very well, it kicks ass,  and  actually a quite a  stable platform.  Very exhilarating. The bag/kite system takes a little  bit getting used to, but  sailing 420s it won't be a surprise (bag, no chute), other than the size of the spinny.  For  £2000 you'll pick up a good  quality example. See the association - there's a few for sail.... actually one on ebay at the moment (current bid £900).  

http://www.laser5000.lasersailing.com/index.php

all the best

  fin

An alternative worth thinking about.....

- if you just want to burn around and  learn to twin-wire and not too bothered  what you 're  out on,  you can pick up a Topper Spice cheap enough.  (I don't think  you'll spend more than 1500quid,  so you'd save yourself a few bob).

Never sailed one, but probably good fun and  more manageable over a  good range of conditions.

http://www.go-sail.co.uk/spice.asp

One for sail in YnY classifieds:

TOPPER SPICE	GBR316, brand new mast, jib and kite, VGC main and foils, bought as bare hull with spars and foils 2008 and reconditioned to high spec.	£1500 ovno