Not sure about the mouse steering, not something that I've tried.
Do you need it, that depends on your taste and addiction to the hobby.
With AC and PC2 you have two excellent titles, Forza 7 does not bring a better simulation or AI, but it does have very nice visuals and a million cars. It took me more than a day and I now have the steering to my liking, but it does not have the same level of feedback as AC and PC2 - maybe it is possible with more tweaking or knowledge of ffb settings.
When dialed in Forza 7 is fun though, even if the AI is not impressive - maybe be at higher tiers it will get better though, I hope so, but up to now I have only seen more speed, not more skill. There is an aggression setting, but that just seems to make the AI even less aware of your position wrt its drive line. Also the AI does not race against the AI, it just forms a line. There is some rubberbanding.
Stability on my rig is okay-ish, in three days I had three CTDs and an number of lockups. Also the paintshop sometimes fails to load. It is still early, so I expect the game to be fixed and even more optimized soon.
Visuals are imo stunning and the performance is great, the racing can be a lot of fun - I really enjoyed racing the vintage Brabham open wheel. Just now I finished a Truck competition, which was great fun, especially the bad weather races were a visual treat.
Just about to start a NASCAR series, still undecided which car to buy
