I'd like to know whether I'm likely to come across fasteners that need
more torque than the wheel nuts. If so, I may have to buy two more
torque wrenches - one for higher torque and one for lower.
Yes, there are fasteners on many cars that have a higher torque specification than the lug nuts. For example I have a car that calls for 88 ft-lbs. on the lug nuts but the crankshaft pulley bolt is 120 ft-lbs. PLUS another 180 degrees. Some suspension parts call for torque in the 100+ ft-lbs. range.
Having worked on cars and motorcycles for a long time I have found that I need at least 3 torque wrenches. A 1/2" drive one, a 3/8" one, and a 1/4" one. Between these 3 nearly the entire range of likely fasteners is covered. One rule about torque wrenches is that you should avoid using them at the limits of their range if possible. That's because the accuracy of the torque tends to be off near the limits. This is especially true at the bottom end of the range.
A secondary, but related question - do I really need a torque wrench
below 10Nm? Here are some examples of low torque fasteners - specified
to two significant figures!
I also have a "precision" torque wrench that I use on my bicycle where you have very tight tolerances for torque like 2 Nm, 3 Nm, etc. My standard 1/4" torque wrench is not able to be accurately set for 2 Nm. My point is that sometimes I encounter automotive fasteners that are in this range and I've also used my bike torque wrench on those. For example valve cover fasteners for an aluminum engine. As I recall I recently needed to torque some of those at 4 Nm.
The best approach, in my mind at least, is to buy what you need when you need it. That way, over time you build up a nice collection of tools. Unless you're a professional mechanic, you probably won't want to just buy everything you can think of up front.