Yes, the speed sensor is held in by a circlip and does have an O-ring behind it, as shown below.
I believe my gasket set came with a new speed sensor O-ring but I'm not positive. Most likely available at your local dealership. Since mine wasn't leaking, I left well enough alone. Same goes for the shaft seal. I had planned on replacing it but since it wasn't leaking, and I didn't have the proper tools to get the old seal out, I left it alone.
It looks as though you can replace the speed sensor O-ring from the rear without having to open up the entire compressor. I would make absoutely sure though where exactly the leak is coming from.
The low pressure switch pretty much protects your compressor, unless it has experienced the infamous "black death". You'll know when you open it up (even just the rear sensor) if it's worth the effort or not. Look for metal particles, gunk etc.... Most likely though you'll find clean oil inside making it very rebuildable.
Mine looked like new inside with 238,000 miles on it. I was pretty amazed.
Good Luck
