I would keep the line at the 18" depth. I don't know where you are, but if you are in the northern latitudes, the line should be below frost depth, but depth will not affect the pressure or the flow. I think you are wise to replace the line now, but why with PVC rather than copper? I'm not sure about the end connections with PVC. As far as curving, that will not affect pressure or flow either, but for possible future maintance (a leak 30 years from now) it would be most helpful to the person repairing the line if it ran straight from the meter to where it enters the house. If it was my house, I'd run 1" heavy copper from my 1" meter (or 3/4") straight from the meter to the house, and I'd be darn sure it was well below frost depth. Come to think of it, that's exactly what I did 15 years ago. It sounds like you have no expertise in this type of work, so I would strongly suggest you have at least 3 estimates and discuss with the contractor what type and size pipe you should use and the depth to lay it.