Yup, best thing to do is put all money into upgrades first.
Save your game, check the bolt-ons for your character for a certain peak in the
FAQ and the do all the events in that peak.
If you get a bolt-on you like, save game, otherwise load previous save.
It's possible to complete a race without getting a bolt-on, so technically it's possible to complete a whole peak and not have a single bolt-on for that peak. (Not sure about that last thing though)
@Gondee, I thought you only got bolt-ons when you first completed an event, not sure though haven't played in a while.