Your size and intentions on riding the track would dictate a full size MX bike. Your budget pretty much restricts you to an older two stroke.
It will be easier for you to learn on a small engine size, i.e., a YZ-125 or similar. I suspect, however, that you would want to move up to a 250 cc two stroke in very short order.
If you want to make your early learning as easy as possible buy yourself a 125cc two stroke and plan on trading up in a year.
If you would rather buy it once and be done with it and don't mind a little frustration and taking a few extra tumbles then buy a 250cc.
If you are riding trails the likely result of too much power is that you will "loop" the bike when you apply too much power going up a steep hill. It's speed and power can also get you in way over your head so just go easy.
On the track you may find that the throttle is a bit touchy. You will hit a bump that will throw you back a bit. Of course you are hanging onto the handlebars but the problem is that your right hand is wrapped over the top of the throttle in such a way that when you get thrown back it cranks the throttle open. This of course throws you back even more, resulting in WOT over obstacles that you were going to fast for in the first place. You may have a few interesting dismounts until you learn how NOT to twist the throttle.
If you survive the first few trips to the track a 250 (like a YZ-250) will serve you well. You should be able to find a decent bike that is 5 to 10 years old in your price range.
Rod