Parts are interchaneable between each kits.
As for the Factory Kit, the chassis isnt super stiff like what lee has said.
The upper deck comes with a stiff design. If you wish to make it more flexible, there are 2 more softer version of the upper deck.
I have run with the stiff upper deck and dont find any problems with it. Handling wise that is.
So if budget permits, get a T1 Factory Kit (not T1 EVO2 Factory Kit). If your pocket is tight, get the T1 Raycer.
T1 Raycer is the same as the EVO2 FOC only it doesnt comes with the pivot ball suspension. Furthermore all the graphite parts has been substituted by composite. You will get the tub chassis, front and rear suspension holder and the upper deck all in composite (some kind of hardened plastic i think).
Just my 0.02