The flywheel may not be needed, you won't know for sure until the car is apart and you see how worn it is, the gel between the 2 flwheel plates also deteriorates over time and heat cycles...... It's preventative maintenance.
If they don't order a flywheel in for the day of fitment, you may need one,, if the flywheel fails down the line, you will have to take it all apart again at extra expense.
I assume all parts are guaranteed for 2 years, so you have that peace of mind?