KTMs, of any type, cark it like any bikes do, especially if you don't take care of them. Or, are just plain unlucky.
There's a nicely

blown up 350 in the 4t section, right now - it looks Very Expensive to fix.
It's rare not to find complete rolling chassis in KTMtalk 's classified section.
Last week , I'm sure I saw a couple of 2010's, for about $1400 / $1600.00
At this moment, there is a 2011 250 /300 PDS chassis, minus forks, brakes and wheels. Usually , you can get entire rollers.
There's a fair few blokes there that seem to make a living selling KTM parts.
Trouble with Old 500 / 540s, is it's a reverse engine, You'd have to mess around with swapping sides for sprockets and brakes. But, like most things, it can be done.
4T and 2ts, have different swingarm to engine widths - I can't recall which is wider, or narrower, off hand.
I've just got a full front end, and a TTX shock, for a great price.