As with any engine the life expectancy will depend heavily on maintenance and proper use. To maximize the life of you machine you should either run ethanol-free fuel (if available) or use a fuel additive to helps protect against the effects of ethanol, keep the engine cooling system clean and free of debris, keep the air cleaner clean and always allow the engine to warm up and cool down before and after use. The list will go on and on about all of the things that you can do to keep it going, but dirty air coming in degrades oil faster causing premature wear in the engine internals and bad gas and ethanol wreak havoc on the carburetor and fuel system.
The Kawasaki engine in the S240 is the same as the one in the X350. The FS series of Kawasaki engines is better than the FR series seen in many lower end ztrs and riders. The Briggs Cyclonic is just an Professional Series engine with a better air filtration/debris management system. They do say that the Cyclonic which is part of their "Commercial" line will last up to 50% longer than the Intek line found in lesser mowers.
One small thing to consider, not to persuade you away from the S240, but the Accel Deep on the X series offers an exceptional cut and also provides the option of a mulching system that can be turned on and off with a switch. This is obviously not needed if you bag all the time, but a good thing if you gave up the bagger altogether.