You wont get away with t posts every 15feet. Here on my exterior fences i run 2 t 1 4to5 inch wood post. I dont use green treat posts, i use creosolt posts. I have posts in cement that are 20 years old my grandfather set, like new.
The biggest thing with your posts is go to the junk yard and get some old hub caps to put over the tops. Will help keep them from rotting.
As for doing your long run of fence, i would suggest go to your local farm supply store, buy some orange bale string. Set your first corner post. Then stretch that string out so you know where you want your fence to be. Best way for setting your steel posts or
As for how tight do you want your wire, it depends on what wire are you going to run. If you are going to run the 12 1/2 gauge heavy duty wire you can stretch it tight. Do you have an atv with a winch? I use a winch to tighten all my wire. I also pull all my wire off of a roller on the 2inch hitch on my quad.
They make a wire clamp that has a big hole in the end of it for hooking to a block and tackle or a winch. Now the only thing with using an atv winch is for your top runs its not high enough. I take a 4inch wood post with some chain attached to it, at different heights i want my wire. Then i attach the wire calmp to it and to the wire. Then use the atv winch attached at the same height as the wire. This allows me to hold the post and draw the wire up tight. Nail it to your end post then go back and start to staple and run your clips.
I have had to run some long fences by my self and i can tell you having a mechincal advantage on pulling your wire and running it will save you a lot of pain and time.