I've not ridden '79 RM100, but it seems to me that the power delivery on a non-powervalved two stroke race bike might not be beginner friendly. One of my friends had an old '73 TM100 (predecessor to the RM) and the power was on or off with that bike.
If you are determined to go vintage, inexpensive and low seat height, how about a dual sport like a Suzuki TS, TC or DS 100, 125, or 185, Honda XL 100, 125 or 175, Yamaha DT 100, 125 (with electric start), 175, or Kawasaki KE or KD 100, 125 or 175.