@ 6'4" 190lbs your technically too big for a big bike.
Personally you would be wasting money dropping money on a KX80, their small.... If you got such a good deal it will just be time and effort to sell it for more and find a big bike. As above a 125 would be the very least (I race a 125 @ 6'5" 240lbs, not the fastest but it's fun)
Ideally if you are very new to riding do yourself a favor and buy a good bike to learn on. 125's are good to learn how to go fast, but not the best for beginners because the constant need to keep it in the correct gear and power band sometimes discourage new riders.
If you can try looking for a trail bike (dodges rotten tomatoes from the moto fans) a KDX200 and the KDX220 are amazing bikes for first timers, they have awesome bottom ends and a very controllable power, but still offer more then enough power for when you get better/more comfortable. In addition you can find clean well maintained ones for under $1500 and good riders for under $1000 all day on the west coast.
Other good beginner options would be KTM's 200, Yamaha's early 90's WR200, WR250, (Much harder to find but they are out there for cheap) Suzuki's RMX250, Late 90's early 2000's KTM 250/300 90's husqvarna wr125/250 were good bikes. Quite a few options out there if you search around.
If you are on a budget don't be afraid of older bikes, a 1991 kx125 is going to be plenty tough enough for an average rider. It's not until you start getting older then the 90's does reliability go backwards.
