This is what circlips look like...
...and one of many of the tools that does the job...
But I don't own one. You can use a couple of pointy instruments to do the same task (I used my gauge cluster pullers).
You're low level light won't come on unless the coolant level gets on or below the level that will trigger the light, or excess coolant manages to penetrate the contacts on the switch, causing false readings (a very common failure, the symptom mine had).
I don't know if you can just buy the seal, but if yours is original, chances are it will give you problems down the road. For all the trouble to replace the seal, I would just go ahead and do the sensor too...the kit comes with a new seal.