What records should I make of underground infrastructure, before covering it up

I would want a good separation between the electrical, water, gas, and any other conduits. I doubt you'd be able to push a water line through the conduits anyway unless they are using plastic pipe for the main, and even then it would be a challenge. In terms of what you may need in the future, here we have electrical, water/sewer, gas, phone (copper and fios), and cable all running underground, but this will vary by area. All the low voltage stuff (phone/cable) can be run in the same conduit.

For a real world example, looking at the construction site next to me, the sewer and storm water lines were buried first because they depend on gravity. Next, they ran a load of conduit right by my home with what appears to be electrical on the bottom. I believe the water lines are running on the complete opposite side of the road. The electrical conduit was encased in cement (this is a large site, so I don't think they want to risk anything messing with these lines). For all of the 4" or larger conduit, they mostly use pre-curved corner joints rather than bending a straight piece.

In addition, there are a few things you may want to use from this site. Above every conduit, a few feet higher, they run a plastic ribbon indicating what is buried below that gets pulled up by any trenching equipment before the line is hit. Every 50 yards or so they have access panels where they can splice the lines running through the conduits and make it easier to pull new lines. And access panels are physically separate for each type of utility running below ground. These panels are 5 sided boxes (bottom is open) that are buried to have the removable top flush with the ground, and the conduit makes a 90 degree turn up to the box and the next piece of conduit makes a 90 degree turn going back down and continuing to the next panel. The more curves, the closer you should space these.

Edit: I just noticed that you said 150', for some reason I had it in my head that you were doing 150 yards. Ignore all my concerns about access panels for only 150'. But like shirlock says, conduit is for the wiring, not the plumbing and piping, and I don't think I made that clear enough in my answer. And for only 150', I'll leave it up to you whether you're worried about forgetting where the lines are buried to want to run the indicator tape above the conduit. It's likely overkill, but the utilities aren't responsible for marking stuff after their demarcation.

I would contend that the gas meter was installed in the wrong place. It clearly was installed long after the house was built and the original electrical panel installed. I would first ask the gas utility to move their mater. I would state that a 'home inspection' found the situation a hazard and are now asking to have the gas meter moved out of safety concerns. In fact, 'my' home owners insurance may become void without reparations. It doesn't hurt to try. They certainly have more resources to make this kind of a change than you do; not to mention the fact that they are the ones that put the meter is a hazardous and non-code compliant location. If you wanted to get hard nosed about it, I am sure you could find a lawyer who would love to help.