As we know, rFactor 2 at the moment runs just with 2 cores/threads, so it is pretty much better to focus on clock speed either with cores count. So from here, there are 2 considerations to do, related on rF2:
1. New 8th gen intels CPUs will run this game worse in comparison than 7th gen, as 8th gen lowers a bit the clock speed to give more cores/threads which will be wasted (in case of top i5 unlocked, 7th is 3.8GHz with 4 cores, while 8th is 3.6GHz with 6 cores);
2. At same price, you will probably find i3 to have higher clock speed than i5 and i7, because the first use less cores and focus more on clock speed, while being cheaper. So for rF2, that might be a to-go option.
But a question while build a new PC, is: what will you run on it? Because if you go just for rF2, the above is true. But for ANYTHING ELSE, you will find yourself with an almost low-end CPU pretty soon, even if you get 8th gen i3 (quad cored now). This because many other games and softwares use way more cores than rF2, while needing lower clock speed. For example, I heard World of Warcraft can use up to 6 cores now if the CPU has them (although is unable to get efficently use of the hyperthreading). Assetto Corsa runs on 4 cores and with the new Competizione releasing soon, we still don't know if they will increase the possible core usage.
About GPU I think the minimum atm would be a GTX 1060 / 980, not even look for lower than this.
