I am interested in buying a bike from a friend. The only thing holding me back is it has a Shimano 105 39/53T Chainring.

I really need to minimally be using a 34/50 compact or a triple with a 30.

My question is, what are the costs associated with changing that out? I assume that it would involve a change of the 105 front derailleur to go triple. Can I simple change out the rings and not the cranks if I go for the double?

I know my friend will give me a deal, but I do need to consider the additional cost.

Thanks!

Richard