I don't know how much you will save by buying a kit, but you should be able to buy all the components at your local farm store. Basically, you will need a spool of visible electric rope or tape, some posts, a few splicers or end stringers, charging unit and a couple of grounding spikes or rods. A kit may not give you enough posts or wire, or it may be plenty.
I have 8 acres and keep about 30 or so step in posts to use for cross fencing or temporary grazing. I keep an extra spool of electric rope on hand (660') for repairs on my existing fence or for temporary fencing. I often have 2 or 3 cross fences going at once. I also keep extra splicers so I can cut and piece together strands to fit what I want and connect the temporary fence to my permanent electric fence.
Regarding end posts, ideally you want to use sturdy permanent wood posts or similar as they will support the tension in the electric strands. You will also need to attach the ends in a way that doesn't ground out the electric. I think they parts are called end strainers.
For the fencer, just be sure to purchase one that is large enough for the distance you want to cover and ground it properly. You can get fencing units that sre solar (most expensive), battery powered (battery not included), or plug in (least expensive). Also check to see how weather resistant it is and if you need to cover it.