Hi Matej,
There is no arcade mode, the components on the cars (mostly) affect it in the same way they would a real car. For example fitting a larger front wing will increase the co-efficient range of downforce and drag. The editor will keep you updated with the important numbers... So, in the case of wings and other aero parts there will be a difference in the ranges that can be adjusted with the car set-up. You can have a car built for Speedways and a car built for tight road courses.
Other elements will have other benefits and penalties (e.g. side pods, exhaust) and I said mostly because yet others will have no physical effect (e.g. mirrors, aerials, camera mounts).
The system is being designed carefully so that there are a lot of parts with similar characteristics to allow for a good variety of say 'short track' cars that will look different, but have similar ranges in their performance capabilities. It's one of the reasons for the large budget, we need to make literally hundreds of car parts to make the system work how we intend it, and it's also the reason for starting with a rolling chassis rather than a bare workbench. Some things will be 'set' such as the engine, minimum weight of the cars, drive-train and suspension components. The diff and suspension settings can be adjusted, but there are no lightened flywheels, turbo chargers, chassis lightening etc etc so the performance of each chassis type will fit within tight parameters.
Having said all that there will be a set of 'standard' cars for each chassis (which will be defined during alpha and beta), with race hosts and championship managers able to specify that only the standard cars can be used... or only cars with parts within a certain performance range... or only a sub-set of parts... race hosts and championship creators can even specify exact designs and setups to ensure all cars are absolutely identical if they wish.
The central idea is to be free and open. We provide the frameworks and the structures for creating cars, race rules, championships, race teams and let it loose to the racers of OpenWheel and see what happens. Tweak and adjust when needed to maintain balance maybe, but put the whole thing firmly in the hands of the community. There may become 'best' car designs for each track - finding that out could well be part of the fun.
The standard cars perform two roles. Not everyone is going to want to build their car for sure, and so the standard cars will provide a quick 'in'. Car builders can share their designs (and setups) as well, so as time goes on we hope that a large library of 'finished' cars can be created. How cool would it be to build a car and have it taken to heart by the community as 'the' car to have for a certain track or event?
Anyway I'm waffling now..