$300 is a sweet deal if the motor is working.
I had a 1978 DT 125 growing up, it didn't have the same style paint so I'm wondering if that isn't a 1979 instead? My 125 had poor carburetion though, nasty flat spot around 4k rpm's I couldn't get rid of.
Mine looked like this one..
"Tough times don't last, but tough people do." Robert A. Schuler