Factory classes are not meant for Average Joe that already has a firearm.
Initially they might work, but factories keep adapting to the rules and eventually there is a shortlist what rifle to buy if you want to shoot Factory and be competitive. If you don't want to do that, I think it's easier to be competitive in Open Light. Unless you have 243 or other 6mm that is allowed in Factory but effectively ruled out in Open classes.
Biggest problem with these Factory rules is that you need to get replacement barrel from original manufacturer (considering somebody that shoots factory, not just tries it time or two). Basically this steers to buy a rifle/system with (QD) barrel change and replacement barrels available over the counter. Second option is to find a manufacturer that does change barrels, preferably in your home country.
Some other considerations are factory options, like interchangable pistol grip on Tikka T3x. Cheekpiece is not a problem, since you can glue or at least tape a suitable piece on. Another European detail that rules are not specifically addressing is set trigger.
Regarding Factory and adding rails on forend, it's a doddle with at least rifles that come with dual sling studs in the front. Technically you could securely attach a contoured rail to rifles with single stud, but I think it begins to be gray area (how large the contoured part can be etc) Or if you want to add full length Arca rail using front stud and front action bolt...