My personall opinion of "is a KX500 a good first bike" is that the answer is flat out NO.
Why? When you are a new rider, you need to learn many skills BEYOND the "bike power", such as #1 being balance, #2 being smooth clutch/throttle management, and so on. You throw a leg over a 60 hp KX500 (which I will argue does NOT like to be ridden "slow" vs. other comprable bikes). The KX500 was designed to do one thing, accelerate and maintain 80+ mph out in the desert. Sure you can ride a 500 in the woods to a certain degree of success, but if you take the same new rider, the same technical woods, and put them on a KDX200/220 or a something like a 250F MX bike, and trust me the learning curve of the rider will be greatly accelerated stemming from the relaxed power. I always say too much power can be fun some of the time, but too much power will always get you hurt in the long run.... (just my thoughts)
I've been riding for 26 years, and have owned 11 bikes thus far, and every time I throw a leg over the KX500 and hold it truly WO, I just have this scary grin on my face and think to myself, dam this thing is freakin' got a lotta' balls !!!!!!!!