Simple little donut insulators. TCS or farm and home that sells e-fence supplies. Yellow or white. The poly rope works very well for this.
As for expansion, it is as easy and flexible as it gets. Decide where you want to make corners.......or bends in the fence......and set steel posts at these locations. You then install the donuts.....and run the poly rope through them.
Then before you get too far into the next step, you may want to get out the lawnmower and cut down the grass that is under the pathway you have selected. As in right down tight.....scalp the ground with it. This time of year that will buy you at least 3 months time until grass starts growing, at which time you can start working on it with vinegar/salt/soap mix to keep it burned down.
Once you have made the entire loop you want to enclose, start working to tension up the fence so each of the straight line runs straighten out. That then becomes your fenceline. At that point, you can then install your step in insulated posts, step them in right beside the line on the ground. Then simply slip the poly rope into the clip that is the height where you want it. Once you have added all the strands you want, then go back to the beginning/end starting place. With poly rope, you can simply grab the poly rope by hand and start to pull......the tension will flow through the entire run. When you got it where you want it, tie it off.
Connect all strands with a short jumper wire......start at top and work your way down. When done, all of your strands will be hot. When you terminate (end) at the same post as the beginning, to test it is easy. Test it right where it leaves......and test it again right where it returns. If both are hot, you can be assured it has made the entire loop and it hot all the way around.
And if you want or need gates to get equipment inside the enclosed area, you can have those too. Spring loaded handles make them easy to open.
Didn't reproduce all the graphics here, but all this is included in my e-fence threads.