hey hammy, the KTM 125 SX if anywhere near as good as the KTM 125 and 200 EXC should be awesome mate. I currently have a 2010 200EXC and it is blisteringly fast (I am very much an amateur tho).
Condition is more important than age generally and you really want one that has been looked after properly. I have a 1998 KTM 300 EXC which is still going strong after many years of abuse (currently in supermoto trim) so you should be all right as far as reliability goes. Obviously the newer it is the lighter, more powerful and more advanced suspension etc will be. I had a shot on a 2006 125 EXC a cople of years back and it was a wicked lil bike []