Yeah for $900 that is a pretty darn good deal and the good thing about it is that it is not too heavy of an investment to get back involved with the sport. Like others have said you can get it and ride it for a season and if you feel at that point you would like a newer model you can sell it and update. Since you are only paying $900 to buy it initially, you can potentially sell it at the end of the season and get the majority of your money back if not all of it.
As far as power goes with the older model two stroke bikes, you really are not going to notice a difference with todays models so dont worry about that. The biggest differences are in the realm of ergonomics, suspension, and therefore handling. Heck if you have ever ridden a mid 70's Bultaco or any MX bike for that matter you know the power is not the issue, it is the other things I have mentioned.