Mine is going on 37k & 8 years old and she still runs beautifully. Certainly not a high mileage bike, but by all indications, she'll last another eight years and 37k, if not much more.
The engine is flawless, its a rather stout little mill. All other systems are built for the long haul as well. Just change fluids, oil & air filters, & spark plug. I'm not fastidious about it, except oil.
The only weak link, IF you can call it that, is the tranny. You do need to replace the belt every 20k, deglaze the clutch every now and again, and perhaps a new clutch at 30-40k. All this is dependent on how you ride her. One up, with a light pilot, less so, two up, or with heavy loads, more so.
Two recommendations, though. Always blip the throttle to 4500 rpm when you take off from a stop. This engages the clutch quickly and prevents premature wear.
Second, and this is from personal observations and preference, keep the mechanics stock. I read and hear of so many people trying to get a little more performance to suit their tastes. Some like them, some don't. Many go back to stock. The 400 is a very "balanced" bike stock. IN GENERAL, When you play with one thing, something else gets affected. I think the engineers at Suzuki did a great job with the bike and choose not second guess their work. IMHO, you will get the longest wear by keeping it just so.
(For all those who will write in saying I'm wrong about this mod or another, note that I said this is my personal preference derived from my own observations. I've not said mods are bad, just that it is a tremendous bike as is.)
Good luck. I think you will find it a Fantastic ride.