I purchased a home in 2010 with a Hardy boiler that was a little over 20 years old. The former resident had died in a car crash and the home sat all winter meaning stuff froze. I had to replace pump, blower, domestic water coil, T&P Valve, Vacuum Valve and a small copper feed segment. I also had to clean years of creosote build up out of the firebox. It has been working great ever since.
When i purchased the home the boiler was the only form of heat so i know it got some serious use (unless the former resident was a polar bear). I added electric baseboards as a backup but never use them. I have only had one issue since the initial fixes and that was with a polybutylene fitting inside my house rupturing.
As long as the thing isn't leaking i wouldn't see much reason to worry. Pretty much all the individual parts on those things are replaceable and yeah if you upgrade you can simply put in the new system and connect up the water lines, just make sure to get a pump sufficient enough for you needs.