I went for a long, long, long, long, long time before I had the light seals replaced and never had any trouble with light leaks. YMMV. But I immediately changed the mirror bumper when it began to deteriorate. This was for several reasons. First, a deteriorating mirror bumper will change your focus point. Not much but it can be enough when shooting wide open to cause some problems. Second, the mirror will stick up sometimes which is a pain. But most important, if the mirror bumper starts to fall apart and the small, sticky pieces get caught in your shutter...I don't think I have to go on as I am sure you get the picture. The result is not pretty.
Beyond that the K1000 is about as bullet proof as they come. It is so simple that there really is not much that can go wrong. Even the battery is optional. I shot mine for years with no battery at all using just the Sunny 16 rules. Where I lived at the time there were not many stores handy, the Grocery Store was over 80 miles away. When I did make it to a store I didn't remember that I needed a battery, and when I picked up the camera and remembered the battery, I was nowhere near the store. No internet then and mail ordering a single battery just didn't make sense.
The split image is nice, but now that I am older and my eyes are not all they used to be, I find the simple microprism circle seems to work better for me.