You can do all maintenance yourself if you want to, and if you don't mind reading the shop manual. If the valves haven't been adjusted at the specified intervals, or if you don't know the history of the valve adjustments, it's definitely time to check them and adjust if needed. Information about it is at this how-to, under "engine"
https://faq.ninja250.org/wiki/Ninja250_Howto
Sorry, I can't help with your seat question.