Is it a good investment; will it retain its value?
Ninja 250s have a very good resale price. You won't lose a lot of money, especially if you buy a used one. They are in demand, sell quickly, and you can get almost all of your investment money back. Of course, new vehicles always depreciate more, but the EX250, if kept in good condition, won't lose money as fast as more expensive bikes or cars.
As with any investment, the condition in which you keep it will have a large impact on its resale value. If you crash the bike, or leave it to sit in the sun and rain, or let the chain rust and stretch, or run the tires down to bald, its value will be negatively impacted. Performing regular maintenance (detailed elsewhere in this FAQ) and keeping it in the best condition you can will help keep your resale value up.