Pretty much the rule with detail from a design standpoint is "As much detail as is still fun"
So far engine design is fairly realistic, but with some simplified stuff, as an example you pretty much just have a "Mild to Wild" duration slider for cams, plus a slider for cam lift (which is basically a power to valve train reliability tradeoff)
Reliability is affected by things like how highly stressed the engine is compared to how strong it is and that sort of thing, and indeed long stroke motors will be more highly stressed when turning the same revs
Suspension design will most likely be a bit simpler as its just not as fun as engines, basically choosing a suspension design, then some springs and dampers for it, perhaps with some suspension tuning sliders.
We pretty much aim to cover every aspect of the car industry that will interest people, watch the website for updates as we work things out
P.S: Join the forums and post suggestions there damn you all!