Gunsmiths often change barrels, but I wouldn't recommend buying a first rifle with a bad barrel thinking you will change barrels some time in the future.

The action is the basis for all guns. Everything else can be changed. Even the government knows that, so that's where the serial number is stamped. Custom gunsmiths will ofter buy used rifles just for the actions, and built a custom rifle up from the action.

Some mods are simple and you can do them yourself, others are best left to a competent gunsmith.

Most rifles don't see any more changes than scopes and maybe recoil pads. Other changes are only limited by your imagination and your bank account.