I've had my 57 plate SE-L auto multimedia for around 6 months now, after changing from a Focus ST. I absolutely love the car and can't fault it at all so far. I got mine from a Lexus dealership for peace of mind with the warranty etc.
I would have preferred a facelift if push came to shove, but at the time the best deal I could get on an equivalent facelift from a dealer would have cost me around 3K more. The car was 1 year newer (58 plate) and as Normski pointed out, I would have added a USB port, bluetooth audi and indicators in the mirrors. These were things that would be nice to have but were not a deal breaker and certainly not for 3K difference, especially when I thought I was getting such a good deal on this one.
With a 15K budget however you should easily be able to pick out a stunning facelift from a dealer and probably receive change so I would definitely go facelift in your position. :D
These two look rather nice at the moment:
http://usedcars.lexus.co.uk/used-IS/IS-250/2.5-SE-L/Woodford-Green/1760689-604572235-1607027.aspx?srcmdc=se_na_re_
http://usedcars.lexus.co.uk/used-IS/IS-250/2.5-SE-L/Bristol/1760689-604581768-1607058.aspx?srcmdc=se_na_re_