Look in the "\Program Files\1C Company\4x4 Hummer\Data\Cars\" folder to see a group
of directories. They correspond to the following vehicles. 

Starting vehicle: "1A1" folder 
Starting vehicle: "2A1" folder 
Starting vehicle: "3A1" folder 
Starting vehicle: "4A1" folder 
Chevrolet Avalanche: "AVL" folder 
Cadillac Escalade: "CAD" folder 
GMC Canyon : "CAN" folder 
Chevy Colorado: "COL" folder 
GMC Envoy : "ENV" folder 
Hummer H1 Alpha: "H1A" folder 
Hummer H2 Pickup Truck: "H2T" folder 
Hummer H2 SUV: "H2V" folder 
Hummer H3 SUV : "H3V" folder 
Saab SUV: "SAB" folder 
GMC Sierra: "SER" folder 
Chevy Silverado: "SIL" folder 
Chevy Tahoe: "TAH" folder 
Chevy Trailblazer: "TBR" folder 
GMC Yukon: "YUK" folder 

Use a text editor to edit the "cars.scr" file in the directory that corresponds to the 
vehicle you wish to change. The start of the file should be similar to the following 
lines: 

//
// Script with car definition
//
// This file is included by Data\Logic\%Game Mode%\Cars.scr which includes 
// Data\Logic\Common\Car.def
// so it can use keywords declared in Car.def
//

Find the following lines in the file: 

PowerTransfer( 1 )
Group( "GM" )
BuyPrice( 57224 )
SellPrice( 28612 )
Power( 452 )
Status( "1" )
Icon( "CAD1" )
Weight( 2570 )
SetParam( "m_nTorque", "758" )
SetParam( "m_nTorqueRPM", "3171" )
SetParam( "m_fMaxRPM", "6548" )
BonusPointsCost(50000)

You can change the values as desired. By decreasing the BuyPrice value you can make
the vehicle more affordable. You can also increase the SellPrice value of the same 
vehicle to make a very large profit selling them for easy money. The PowerTransfer 
value should be kept between 400 to 750000, and corresponds to the horsepower. 
Decreasing the Weight value can make your car fly when driving over low ramps, and
represents the weight of the vehicle in kilograms. The m_nTorque value represents 
torque in Newton meters and can be increased. The m_nTorqueRPM value can also be 
adjusted, but the higher you set the rpm, the slower the truck will produce power.
Also, the m_fMaxRPM value represents the highest speed at which the engine operates.
Make sure the m_nTorqueRPM value is lower than that. 
Change the BonusPointsCost value to set how many bonus points are required to obtain
that vehicle.