It sounds like a terrific deal! The bike is worth about twice that so I don't see how you could go wrong. If "all the gear" includes a helmet and other riding gear then the deal is fantastic, especially if the gear fits.
Just for reference, here is the gear that you should have
Helmet: $130
Boots: > $100
gloves: ~ $20
goggles: ~ $20
optional, but highly recommended:
Chest protector: $50
Knee pads $20
elbow pads $20
riding pants: $ 50 and up (and up, and....)
Jersey: $ $20 and up
So, if you got a running bike, helmet, goggles and boots for $800 you just can't go wrong!
As for the suitability of the bike: You are plenty big enough for a full size bike. The 250 will have enough power to get you into trouble so just learn to NOT twist the throttle all the way and especially learn how to "untwist" the throttle when you have been thrown back on the seat. Once you learn that you can tame the bike. Based on your age and prior extreme sport experience I would expect you to not have any trouble at all.
The only trouble I would expect you to have is perhaps on the first ride or two. Two strokes do not like to "put". If you are riding a bit timid and not allowing the engine to work hard it is common to foul the plug. Once you get to the point where you are routinely getting it "on the pipe" and letting the engine get up to full temperature you won't have any more problems.
Rod