I have so much electric fencing in my paddock think maybe OH at least might class as a guru lol.I have paddocks,track systems,spring gates,schooling area,you name it I have probably done it lol.I also have fat pony so feel your pain,have you thought about a track system rather than a paddock,keeps them moving more I find.
Anyway to make a paddock I would strongly recommend you use at least a couple of wooden stakes in addition to the plastic posts,if you have any corners you need to have proper posts,plastic ones will not take enough tension to have an effective fence that will cope with wind.If you are going for a circular arrangment I suppose you might get away with it,but especially if you are having a gate way you really need something stronger to tie on to than just plastic posts.If you are using wooden stakes (or maybe tie onto an existing fence is you can't bang in stakes??),then you will also need plastic insulator rings to screw in to the stakes and put tape through.
Other than that I think you have it all on your list.I got me energiser from mole valley for £70,is excellent and I run it off a car battery,cheap as chips and stays up for weeks in between charges,gives a hell of a belt as well:wink:
Don't forget your gate handle will need to connect to the fence in some way,not just hook onto a plastic post,I would get the proper metal gate rings for this (can't think of their proper name but someone else will know I'm sure) and screw into a wooden stake,but can also use tape wrapped around a pole or insulator (I have done this recently on a temporary bit of fencing and works fine),just make sure it's touching for a connection.
Make sure your earth pole is down as far as poss and damp ground is best if you can site it somewhere suitable for that.
Really will shut up now lol,but hope that helped and that your fencing turns out well!!
its not a massive run either - a double strand dividing one smallish field., with nothing shorting it out. 
