You can rent a post hole digger to help with the holes. There are two man "X" type ones and another that is towable and has two wheels that pivot down with the bit. You want the towable one, especially if you have rocks. The X style will feel like you're getting hit by a dump truck when you find those bigger rocks.
Its really not that difficult to set the posts. Run a string line where you want the fence. Mark the hole spot based on what the fence people tell you, then dig the hole. The fence places deal with DIY-ers and will explain the process to you as well. I have no doubt that you can get this installed and looking good, just based on your welding and attention to detail there.
I always set the posts in dry cement mix. With this you put the post in the hole more or less where it needs to be, fill the hole with dry mix, make the adjustments so the post is square and at the right height, then water them in.
My one neighbor had a skid steer with a post hole bit we used for my one side and then the line on the other side of my neighbors house. I did my north property line later and dug it all by hand using a post hole digger and a long pry bar. I found a boulder the size of a city garbage can right where my post needed to go. I dug and dug and dug until I gave up and had my neighbor come pull it out with his machine. The rest of the holes went without much trouble. Had I spent the money on a rental I could have dug them in a couple hours. The neighbor on that side is an older guy so I didn't ask him to help other than to split the cost of material.
See if you can get your neighbors to help with the labor. Or if they aren't willing to do that have them rent the machine to dig the holes.
I do most my digging with a 5' bar and if it can't make it through I fill the hole I got with water and try again in an hour. 