My brother rides the '05 450EXC and it's only his second year on a motorcycle. He manages just fine, and has a blast. I'd say it's easily manageable. The power starts low and ends high. It's not "twitchy" like may be inferred. It has a linear 4 stroke delivery. Good ergonomics, suspension, brakes helps make it easy to ride.
My buddy who had the '05 400EXC put the 453cc kit into his after a few rides, and he like tight trail/woods riding. The shorter stroke 400 motor spools up quickly and is reportedly a faster turning bike (ask Enduro Champ Mike Lafferty about this).
The 525EXC is the same weight as the 450EXC and has just more power all around. It consumes about 10% more fuel than the 450 (or 453) which is the only drawback of the 525EXC.
I'm 160lb and I don't find the525 too powerful or hard to control. I'm an Intermediate skill level rider. Most people that have tried my bike commented on the bike as being "smooth" for power delivery.
All three of us have our bikes dualsported, but I would pick this same bike as my dirtbike if I was interested in offroad only.
Keep in mind that as KTM fans we are just that -"fanatics" aka addicts/junkies. As such weare hopelessly hooked, and will say anything to justify the dollars spent on our habit! So takeall opinions here with a dose of salt.
Since you asked about the 450EXC and 525EXC KTMs I have given my opinon on them. They aren't the only bikes out there and I know that I would have a good time onmy old'86CR125 as long as it has a fresh knobby, a tank of gas, and a riding buddy to play hare and hound with!
Check
http://www.ktmtalk.com for more info on the KTM 4 strokes.
Good Luck floored!