First ATV
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ATV safety course= great idea. Physically you are big enough to ride most anything, but I would stay away from the big bore/open class machines and the heavier full size utilities. Something like a 300EX or Recon would be perfect. Don't worry about the manual clutch, it isn't hard to learn. Heck, if you were into dirt bikes you would have no choice but to learn a manual at this stage.
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I would suggest getting a honda 300ex. They are nearly impossible to break and It has reverse. There are plenty of aftermarket parts. Don't go out to the dealership and buy a brand new quod for 4,000+ dollars because you can find some really nice used stuff on craigslist. With about 2,000 dollars you can get just about any 300ex you want on craigslist. Just don't overpay because there are tons of them out there