If the metal tank is showing signs of rust, and has been poorly maintained it will need treating or replacing. As for the older boiler, yes with correct boiler sizing and selection coupled with installing time and temperature controls for zoning your heating system, you could be looking at up to 30-35% reduction in heating costs (you would also qualify for a €700 sei grant)
As for specific costs, you will really need someone to see what is required (not really more than €3000 assuming the tank replacement is straight forward and bunding is not required, and the ch pipework etc is ok)
Look for a good recommended installer in your area, maybe even look for Oftec registration