I happend to of had a 97 300EXC and now have a 2000 300EXC
I would say the prices on both bikes seem a bit high to me - but if both are nice enough they may well be worth it.
the 2000 is the "next generation" of KTM from the 97. In relative terms, the 97 sorta seems ancient. Not that it is a bad bike however. The 2000 forks are considered some what harsh and a step back from the 99 models, but that can be addressed with a revalve (I did my own with a $40 kit I got off **** - well worth it!).
As to play riding with the kids, that is exactly what I did with both of my KTMs. Just dial in the jetting accordingly, soften up the compression dampening for more comfy kid pace "cush".
I say go for the 00, but talk the guy down on price. No matter how nice and little use it has had, it is 5 model years old...he should have some room on that price. then again..if it really is that like new....??