i'm 16 and love riding but i have been befor i remember (like just cause: https://drive.google.com/file/d/0B2YNAglzrtWBVkZnUDZBRzJZS3M/edit?usp=sharing ).
But anyways i think you should get a nice used 250cc 4 stroke, the reason being is 4 strokes are a bit more complex but simple once you know the basics. also 2 strokes are in EXTREMELY GENERAL TERMS 2x more powerful now it is VERY GENERAl also you don't have to pre-mix fuel, not difficult to add oil to gas... (i know i plan on getting a gas gas ec250). myself i have a yamaha tt r230cc 2007 but for me that is getting a bit small for me and i'm same height as you so that is why I say 250 cc.
DON'T go with a 450, i once saw a group of 5 new guys with massive 450s and it probably ruined the sport for them.
also what state are you in, if you are in MI, as i am, watch out for DNR make sure you do the off road safty test and what not, if you are not in mi look it up.
The Moto series is nice to watch. Some are better than others, but it is nice just watching guys ride in the dirt.
If you have Vudu.com. You can buy the bundle for streaming only at
http://www.vudu.com/movies/Movies_mobile.html#!content/732669/Moto-The-Movies-1-7-Bundle
It is $35. If you do decide to buy it, make sure you click HDX which is 1080p and 5.1. You can buy the discs, but they are much more expensive separately. I don't think you can share this, so I'm not sure ho he can use it unless you buy it under his account. There may be other deals for it somewhere else though.
Look here http://www.motoxcinema.com/ and you can find movie ideas and maybe find them cheaper elsewhere.
Good luck! 🏁