Whilst a BMX helmet will offer at least the same level of protection as a road helmet (which are designed to absorb a certain level of energy upon impact rather than for a given speed of impact), the former has no ventilation and therefore your head will get very hot and sweaty in warm weather on rides of any distance..also applespider if you are only riding your bike to work in the summer and it is less than 18 months old then there is no way your chain ring should need replacing (they should be good for about 10k miles at least). At the very most all you will need is a new chain (but only if it slips/jumps) and possibly new tyres/ brake blocks...