If you go 4 stroke, it sounds like you should stick with something like an XR from Honda. The more recent racing four strokes require a lot more maintenance, but will really perform. Not that you couldn't handle it (the maint) or that it is too time consuming, it's just the caliber of today's 4 stroke race motors - you have to keep a wrench on them quite a bit. Like I said the Honda XR or even Kawi KDX sound like what you want. They sacrifice power and weight compared to the racing 4 strokes, but if that doesn't matter to you then your fine. They are very reliable. As far as dirt bikes go I personally am a die-hard KTM man. They make a great line of woods bikes that are specifically set up for that stuff right out of the box. I would suggest either the 200EXC or 250EXC, both of which are 2 strokes. The EXC line is set up for woods riding, fast and bullet-proof. Maybe take a look at these 2 bikes if the XR or KDX sounds too heavy and you would want more power. If the KTM interests you there is a ton of info at KTMTalk.com.