On the subject of KTM forks, rather than brand new WP forks, I guess the question is "how old can you go?", or "is it worth paying more for 2009 ones over, say, 2004 ones?".
I know at some point they changed the axle diameter (2003 at the latest AFAIK), so you'd want to be later than that, but is there any discernable difference over, say, the last 7 years? Bearing in mind they're all going to be miles better than the Yamaha forks.
Older is of course cheaper, and you're going to have them apart to be rebuilt anyway so they'll have fresh oil/bushes/seals as necessary.
Finding a set from one of the bigger bikes would probably be good, but they don't seem to crop up that often, whereas there's always EXC/SX forks on eBay.
On the subject of things that are available, does anyone know if the supermoto variants have shorter travel forks, or if they're just sprung/valved differently? If they do have the full travel of the off-road versions, then a set from a 640 supermoto might not be a bad starting point - lighter bike, but set up stiffer than you'd want for dirt riding. Put that on a heavier bike and it might be in the right ballpark. Maybe
A final thought is that, since practically everything KTM is interchangeable, once you are set up with yokes to take WP48 forks, it's pretty easy then to 'upgrade' to the latest super-wizzy enduro forks, or put an LC8 twin-disc set in, or even go back to OTR for a brand new set.