CS = Carbon Steel
HSS = High Speed Steel
The high speed steel is much harder than the carbon steel and HSS tools generally keep their edge much longer than carbon steel. These prices are quite cheap for HSS taps and dies and I would probably question how good they actually are. I believe that HSS taps are thread milled and the milling is not great on some of the cheaper ones. For occasional modelling use, carbon taps are fine. I have some carbon taps that are still doing a good job after forty years or so.
The taper tap is really necessary for starting a thread since the taper on the tap starts the threading comparatively gently with a shallow cut and the cut gets deeper as the tap gets furth into the hole. The second taper has a steeper, shorter taper and might be useful if a lot of shorter, blind holes have to be tapped where the longer taper of the taper tap doesn't allow sufficient engagement to provide a decent part thread for a plug tap. The plug tap has no taper and is necessary if you want to tap a lot of blind holes to the bottom of the hole, which the taper taps wouldn't allow. If you only can have one tap, then the first taper allows you to do almost every job bar tapping blind holes, provided there is enough clearance behind a hole to allow the tap to go through onto the parallel part of its body to cut a full thread. The next tap would probably be the plug tap to deal with blind holes. The third tap would be the second taper for those occasional situations where the longer taper on the first taper tap is a problem. It is not recommended to try starting a thread with a plug tap.
Some makes of plug tap come with a point on the end which was necessary for the manufacturing process but is a right PITA when threading to the bottom of a blind hole. The point can be ground off to avoid this, but should be done carefully to avoid messing up the first thread on the tap.
There is usually only one die to cut external threads and most dies are usually split dies which can be adjusted in a holder to set the diameter of the thread. So some form of die holder is required. The dies also have a plug side and a taper side, the taper side being used to start a thread and the plug side to allow a thread to be finished up to a face.
For thread cutting a lubricant helps a lot and I use Trefolex which works very well. Don't buy a large jar of it. I was given a small amount some forty-odd years ago and I'm still using it.
[Later] I have used this supplier recently for a set of HSS 8BA taps and they were very good. Another advantage is that he is only a few miles from my front doorstep so I can get items in the same day.
BA Taps. Highest Quality. Free UK Shipping.
Jim.