You can TOTALLY modify and upgrade cars on Forza 3. -- Every car will have its limits on what you can and can't upgrade. Here is a list of things that are available for modification on most cars:
Engine and Power: Air Filter, Intake Maniflod and Throttle Body, Fuel System, Ignition, Exhaust, Camshaft, Valves, Displacement, Pistons and Compression, Oil and Cooling, and the Flywheel.
Platform and Handling: Brakes, Springs and Dampers, Front Anti-roll Bars, Rear Anti-roll bars, Chassis Reinforcement and Roll Cage, and Weight Reduction
Drivetrain: Clutch, Transmission, Driveline, and Differential
Tires and Rims: Tire Compound, Front Tire Width, Rear Tire Width, Front Rim Size, Rear Rim Size, Rim Style.
Aero and Appearance: Front Bumper, Rear Wing, Rear Bumper, Side Skirts, Hood,
Conversion: Engine Swap, Drivetrain Swap, Aspiration Conversion
You can also fine tune your vehicle after you've made your race machine as fast as it can go. Fine tuning doesn't cost any money, you just have to know what you're doing. I never mess with this setting myself... Things available for tuning are...
Tires: Front Tire Pressure, Rear Tire Pressure.
Gearing: final drive, 1st, 2nd, 3rd, 4th, 5th, and 6th gear. (Also 7th if the particular car has it)
Alignment: Camber; front and rear, Toe; Front and rear, Front Caster; angle
Anti-roll Bars: Stiffness; front and rear
Springs: Stiffness; front and rear, Ride Height; low or high
Damping: Rebound Stiffness; front or rear, Bump Stiffness; front or rear
Aero: Downforce; front and rear
Braking: Braking Force; balance, Braking Force; Pressure
Differential: Front and rear acceleration and deceleration, Center torque
Hope this isn't overkill.
There are about 400 cars in forza but you can get a extra 90 by dlc
yes
More cars and tracks probably
no
purple is my favorite number in alphabet
go to edit car
500 cars
1300
There are no nitros in Forza 3.
Forza 3 has 1 player online, or 2 player split screen. In split screen you can have computer controlled cars racing too. This is one reason I like Forza games, as games like NFS Shift, Grid, Pure, Dirt, and Dirt 2 don't have split screen. Trust me, I have Forza 3. =D
You can't
To sell cars in forza horizon one must first have a properly register account. One must then enter as many details about the cars they are trying to sell as possible.