The 4 : 1 coax balun is the simplest. Make sure your
balun designs are specifically for 70cm, rather than
trying to adapt HF designs.
The impedance of the folded dipole depends on the
environment, as well as the ratio of the diameters of
the two conductors. If you are backing it up with a
reflector to get more gain, then adjust the spacing
to get a 200 ohm Same if the antenna is mounted
beside a metal mast.
As the driven element of a yagi, design the yagi for a
12.5 ohm impedance, and the folded dipole will step
that up to 50 ohms so you can use a 1 : 1 balun.
So the rest of your antenna system and environment
will affect the feedpoint impedance of a folded dipole.
And, if it still ends up with an inconvenient impedance,
then you can use a thicker conductor for one side of
the folded dipole to improve the match.