you say they are having to replace their systems as they're 18 yrs old but we regularly go to boilers/systems that are 30 to 40 years old and they are still running strong, is just a case of having them serviced once a year and having them repaired if they break down. most old appliances can be kept running as long as the spare parts are still available, when the parts are no longer available then yes it may be a good idea to consider a replacement. the only other consideration is having a more efficient boiler to reduce your gas bills.
any installer you ask for a quote is almost certainly going to suggest you go over to a combi and to rip out your cylinder and tanks, simply because they are quick and easy to install. the usual preference for the new boiler is the airing cupboard as all the necessary pipework is in place and would just need adapting to fit the new boiler.
you could have a traditional boiler still (condensing though) but to comply with the latest energy efficiency regulations you would have to have your cylinder upgraded to the latest energy efficient high heat recovery coil type, you have to have thermostatic radiator valves fitted if you dont have them and your pipework between the boiler and cylinder will need to be lagged, also your controls may need upgrading too, this all adds significantly to the cost and is why most people have a combi.
i dont know much about power showers but you can certainly have a thermostatic mixer shower run off the combi although we usually reccommend an electric shower as they are a great back up if the combi breaks down and you've no hot water.