How much to replace a wall mounted gas fire boiler?

[Joe Xie]

I am about close escrow for a unit with wall mount gas fired boiler providing hot water and water radiator for heating. The seller disclosure that the tenant called the shower sometime do not have hot water but could recover itself later without doing anything. My question is how much it would be fix issue like that? How much it will be to replace it, the new boiler and how much to install? The property is located in Bay Area CA, the current boiler is Baxi's Luna 310fi. Thanks in advance for you inputs.