Once the return lines have been replaced, they last a long time. Even then, at most, they leak a bit. That's enough warning to get them replaced. Not so much a worry. I would have at least one plug in relay. And the switch, (the one that LOOKS like the relays). I am a hoarder. I would have lots of things squirreled away. But probably never use in 1000 years. If you're S1 switch and S6 switch are the Mark 3 type switch, and you have the Quad circuit fuse installed, you have covered a lot of ground. The S1 was a big problem. New switch, you never need to worry again. Lake Jamawieb say, water pumps and starters are normally long life. A volt reg? Well, I know I would have one, extra, but it's not like they go bad once a year. They are in principle and practice, long life also. What we need to hear from, is people who run these things ALL THE TIME. Like for work. I don't think I would have many, if any, engine parts. If you change your filters and oil in a timely manner, change your antifreeze also, the engine will last a long, long time. Mike's post says it all. If you have some gold dust saved up, buy what ever you can afford. But I don't think you need to have enough parts to build a new set. Oh, I would have a mag pickup also. Yeah, that part has also a long life, but if it gets loose, or someone incorrectly installs it, you are dead in the water. I believe in todays world, you don't even have to get an army mag pickup. There are generic ones on the market. As long as it fits the hole, you should be able to use it.