thinking of getting a 07 crf250r are they reliable? dont really no much about 4 strokes maintenance, how often do i change the oil, clean air filter etc? and is there anything else major i have to look out for?

I would change the oil and clean the airfilter after every ride.. then oil filter and tranny oil every other ride... then if you are riding mx but without reving the bike out too much then bottom end every 100 to 150 hours, piston/rings every 50 hours. And vavles will pry last about 80 to 100 hours before they go out on you

not too bad of maintenance make sure you check your valve clearances!!if you dont shim your valves when they need it they will wear out faster.
i do a top end when my valves get close to running out of adjustment.

clean the air filter after every ride and if you use expensive oil in the engine/tranny you can go for a few rides then change. you can let the tranny oil go for a little bit longer before changing too. put new oil filter in every 2 oil changes.