Your hot-water demand, existing pipework and available space all influence the choice. A combi heats water on demand; a system boiler works with a hot-water cylinder; a regular boiler typically works with a cylinder and separate feed arrangements.
Frequent faults or changing household needs can make replacement worth discussing. Age alone does not settle the question. Compare the condition of the boiler, a repair estimate and the work involved in a replacement.
Ask what the quote includes: the appliance, any pipework or flue changes, controls, commissioning and handover. Confirm the manufacturer’s warranty terms for the specific appliance and installation.
Further reading: Energy Saving Trust: boiler types