Your post is difficult to read, but I'll do my best to give you some advise.
No bike is a "time bomb" if proper maintenance is performed. Different bikes require different types of maintenance. For example, a two-stroke will require more frequent replacement of top-end components (piston, rings and bearing). However, the replacement is easy and fairly inexpensive. On the other hand, a four-stroke requires close attention to an inspection schedule. When parts need to be replaced, the procedure is a bit more complected and more expensive due to the added complexity of the engine (valve-train).
If you are looking to purchase a well maintained KX250, it should give you hours of reliable service. However, I would suggest when purchasing any used bike that a top-end inspection be performed. This way parts that have reached their service limits can be identified and replaced, rather then waiting for them to fail. When components fail, you will be looking at well over $1,000 of repair whether the bike is a two-stroke or four-stroke.