I'd suggest a two-stroke.They are pretty easy to work on and fun to ride.The most reliable bike I have ever ridden is my old wr.It is indestructible,no joke.The wr's are awesome for the trail and have better suspension than the other trail bikes.yz's are really nice and handle better than the wr's,yz's are my personal favorite of the race bikes.
If you don't think power will be a problem,go for a 2-stroke 250.I have always found yamahas to be good and realiable,but any kind will do,it's just preference,or even just a colour choice.
Good luck,hope you find a nice bike.