Here's what I tend to do if I go about things seriously...
Find an offline race, any one will do. Run that race a couple times bone-stock, no adjustments, nothing. I usually take a mental note of my fastest lap, for reference later on of course. Then I look at upgrades but only the truly beneficial parts. To me that's adjustable trans/suspension, plus a CPU tune & racing filter. All the normal stuff that is most likely to be done in the real world, like the clutch/flywheel as you mentioned. Afterwards you simply run races and adjust/tweak until you can't get anything more outside of overall driving style. Theoretically, if your car is on par with everyone else, you should be able to do decent with factory settings - adding parts won't make you a better driver.
Truth be told, you can also buy everything which could be removed down the road. That means stay clear of the engine/chassis work and race-mods (individual aero-mods can be removed, but it costs $$$). This route probably gives best of all worlds depending on mood... stock, slightly modified, or balls to the wall 👍