If your going to ride a 4 stroke it needs to be at least 500. If you want to move serious dirt a Husaberg 650 is the go. I'm nearly 50, unfit and punt a 07 Ktm 525 through tight enduro tracks no problem. As for top end rebuilds, around $1,200 -1,300 including labour. On average usually around the 5-7,000 klm mark depending on your maintenance schedule and how you ride.
I've ridden four strokes all my life. Well, I actually rode two strokes up until Yamaha bought out the TT500.
Two strokes will produce more power with less weight for the same capacity and in general are probably a better weapon. But I hate a bike that sounds like a lawn mower, and I love the low down grunty torque that you can only get from a big thumper.
Think... filleting knife versus meat clever.