Cdi costs more money unless you already have a Cdi, but you could then spend more money to get a lighter flywheel.
If you are building these bikes to ride together you may need slightly different builds considering the difference in rider weight.
If you are going to tear down the motor and choose a 43mm kit to match transfers you should be able to have no problem hitting upper 30s with a single speed and still be able to tackle hills. To hang together maybe variate one, even with stock parts.