>>350531$4k seems a bit high for an '04 525 EXC.
Since it's plated (right?) and comes with the second set of wheels (probably $1k alone) it's not a bad deal if it's in good mechanical shape.
That's the RFS engine, same one I've got. Not many changes between that and my 07.
There's a couple of things you can do that'll really improve things:
-Replace the crank bearings with roller bearings (usually done by the sumo guys, so ask if it's had that done).
-Fork bypass mod - basically drill two holes through the inner fork tube. Softens the "hit" offroad.
-Head/valves - Bore 'em out 1mm, and put in Kibblewhite stainless valves. From the factory, you're adjusting valves every other oil change. With the Kibblewhites in there, you adjust them once or twice a year.
EXC's come 50-state street legal, so ther'es some emissions crap that can probably come off unless you're in commiefornia. My XCW is the "off-road" EXC, so it didn't come plated/titled, but it didn't have the emissions shit either. It's easy to get bikes plated here, and I wanted to do the electrical my way anyways.
The KLX's I've rode haven't been that impressive, but I've not ridden one with wheels on it (just snowbike kits). I don't know a whole lot about them.
For sumo/highway use, especially if you're at elevation, you'll probably want the additional displacement of the 525.
Had mine for years and I have no desire to upgrade (I just want it to run right now...)
Lots of good resources on the forums for the KTM's.