I agree with some earlier posts that basically stated that it would preferrable to spend the time racing, rather than trying to figure out what combination makes for a challenging race.
Thankfully we have these great forums! When I sit down to race a seasonal challenge, I like to look at the Forum posts that coincide with each race. I like to see what combination some of the better drivers have said they used to win the race. That sets my challenge!
Unfortunately, each time someone says they won an event while only using "x". someone else always seems to chime in with a comment like "what do you want an extra medal for that?" I wish people would realize that when someone takes the time to post that the race was beatable with only one upgrade in tires and nothing else, that is very helpful to those of us looking for that "ideal" challenge.
I went to a friend's house tonight, he is a casual gamer with no sim experience, and showed him the seasonal events. It worked out perfect, because I already knew what I would need to complete the events, and I was also able to help him setting up the cars for him. We would alternate doing each event. He would choose the first event he wanted to do. He picked a car, that he then (with my assistance) set the car up with quite a few modifications. He had fairly close races eveytime. Some he won the first time by a tight margin, and several others took him several tries.
When it was my turn, I could use fewer modifications to achieve a close race.
So in this situation, it could not have been a better setup.
Those that want a bigger reward for racing with fewer mods, are in a sense getting that, because they spent less money on modding the car.
If we all continue to share with the community which setups worked to win each race, we can all spend more time having challenging races, and less time figuring out what is possible.
Edit: Here is a link to a thread about restrictions for a strong challenge:
https://www.gtplanet.net/forum/showthread.php?t=159986
Thank you to Amar212!