Even galvanized will rust together. I like brass valves but PVC everything else though i like the black plastic non potable line but make sure you rigid Mount everything. Drive a fence Post or rebar or.... and tie the plumbing to it as close to the valve as you can to keep it from flexing as much as you can. This will give it the most longevity vs anything else including material choice.

1. Yes: you absolutely want the outdoor rated PVC if you're getting sun exposure. You can cheat, it's not like the white stuff will be immediately destroyed, but if you want something that will last a bunch of seasons, the "grey" stuff is the way to go. Double check that it's UV rated though, and doesn't just happen to be grey.

2. To get around all of that, you can bury it. Because you're just doing it for the garden, you don't need to dig down to the frost line. Just make sure you clear the line at the end of the season. Another advantage is that you'll minimize the amount of water that's been baking in the sun idle in the pipes. If it's a heatwave and they're in direct sun, that water can get downright hot to the touch. I've never lost a plant because of it but frankly I'm kind of surprised by that. If you do bury, you might consider running some electrical conduit at the same time, even if you don't put wires in it (DO however include a pull cable for later use). What you do at either end of that is a whole other project, but you can always just cap it and get it to it when you get to it. Solar + Battery usual works great for garden automation stuff, but being able to run an ethernet cable can simplify a lot.

3. Plastic will hold up fine, but as others have mentioned you might want one of these.. The union allows to remove it. You could do a more simple threaded system IF you are able to completely and freely rotate everything "down stream" of the valve. I'm just going to say the stupid part out loud because I learned pipe stuff the hard way: A ball valve threaded on both sides cannot be loosened from one side without tightening the other (again, unless that other side can freely rotate). Edit: alternatively unions are sold separately, and sometimes you can eek out some flow advantages that way but it's in no way worth thinking about at garden water flow rates.

4. Finally, a last alternative I've seen done well for gardens that sort of "wrong done right" is putting posts up and stringing a hose over head. It kind of seemed like as much work/expense as burying it, but I guess they had the posts, it came out really sharp in the end. You need a pretty high quality hose though. Baking in the sun and sagging under the water weight can end badly.