Nowadays DSLR rarely fails but if it does, main major repair needed will be failed/roasted sensor (which can amount to a price of a new cam) and next will be replacing shutter (most probably a few hundreds replacement cost).
|D3 Shutter Unit|
To check how long cam shutter can last, check out here. If your model is not listed there, then a quick Google search should provide you with an answer very quickly.
However, one shouldn't get too anal about shutter life. e.g. Nikon D300 has an estimated 150,000 shutter life which ≤ to:
36,500 shots/yr x 4yrs = 146,000 shots
A whopping 4 whole years to even hit that amount and I bet by that time you would have gotten a new one! Start shooting and stop worrying!!!