I don't have a lot of advice here since it is just a fence afterall- but definitely put 1/3-1/2 bag of easy-mix concrete in each post hole so they are super stable. You just hose it down mix it up and bury it and leave it. You need to build the fence as you go (including digging your holes)- if you dig all your holes first they will never line up!
As for staining them / making them look old (this works on all "new wood") - here's a super simple trick that works like a charm and is super cheap to boot-
Mix a plain brillo pad (no soap) with 1/2 gallon of cheap old white vinegar for around 8 hours (just overnight is fine, there is no exact science). You can break the brillo pad up however you need to get it into the vinegar container. In the morning, remove the brillo pad and mix 1/2 gallon of water to the vinegar (note that the vinegar may still be mostly clear or have a grey tint to it). Once you mix the water and the vinegar, stir it up good and that solution will looks like dirty water- but whatever wood that touches will turn grey in a matter of 10-15 minutes. It is super thin, so you just brush it on with a normal paintbrush- be generous with it so it goes in all the cracks and crevices. When you're done- the difference is night and day. Here's my barn I did with this before and after, and then my living room.
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