If it is outside you can probably use black polyethylene. It is cheap and flexible. If you use it, you should be sure to use UV resistant fittings. I have replaced a lot of the white nylon fittings with PVC fittings for poly pipe.
I don't like the connections for poly pipe because of the difficulty with leaking joints so I prefer to use rigid PVC where possible.
Can you run some of the lines rigid and then come off with flexible lines where needed? You might want to consider gray PVC or CPVC. There is also flexible PVC for moderate pressures that would probably be acceptable outside.