It sounds like for your recrational type riding a 250 is perfect. They are hella fast, and there will be some time in the beginning when you're afraid of it and most of your improvements will just be controlling the bike. Once you get that done I think you'll love it. I to started out on a 250, unless you are racing and want to be in the 125 class and 250 is smart because since you hardly ever are ripping on the engine, theres less wear, and less chance of seizure. Just run a clean burning oil like superM, Mobile MX2T, Amsoil and you shouldn't have to rebuild and clean your engine but once a every other year. Though if you ride 3 hours a day, your gonna have to more than that. Also dirtbikes take some care, and parts like the chain do wear out and need replacement, putting that stuff off costs more money in the long run when a chain derails or breaks and damages other stuff. I'm just saying that because you mentioned your limited budget, and to do tons of heavy riding like you said, it aint cheap, buying fuel, premix, oil, and other parts that break. Yeah a 125 has some big balls, but you have to keep up on the maintenace or like your buddies RM they go flat, and it really isnt cheaper for a 125 pistion and gaskets then for a 250s. At least for me, when it comes to my cranium, I dont like taking risks, brain damage don't heal. Get the MX helmet. Personally you don't need a chest protector, thats mostly for getting hit by rocks from other riders, but they do help in trail riding when you run into a branch or something that would normally make a painful scratch.
Good luck,
-Jim