1. Forza 3= 8. GT5= 16.
2. I dont know to be honest. I would wait for confirmatrion because i cant see PD obly putting 16 out of 40 cars on a track. But nothing has been confirmed so
3. PD has stated that they are going to be introducing a store where you can purchase and upgrade the cars internals, like tires, engines ect ect. Even more so then oher GT games. Of course you will purchase this stuf with in-game money called credits.
Erm forza 3 is said to be very customisable when it comes to the look of your car. Paint jobs deco ect ect. I dont know about the internal stuff though,
My own points
GT has always been more realistic when it comes to the handling f the cars. For me Forza games have always been more arcade like with most cars driving more or less the same unlike GT where every car drives like it would in the real world.
Eva way if you prefer the more realistic handling, fine tuning of the engines, NASCAR, more racers on a track at once and crisp graphics then GT5 is for you.
If you prefer to customise your car how you want it to look, and more relaxed handling of the cars with decent graphics then forza 3 is for you.