wohoo, another hifi-freak?
you're right, impendance is not exactly the same as resistance, I just thought it's easier to understand that way. the impendance is a product of the resistance behaviour of a chassis under specified frequencies, and to make it even more complicated it also includes the capacity in context with the amplifier. in reality it is just a theoretical value because they use DC frequencies for the measurement which usually never appear in practice.
well, nowadays usually all speakers (except really exotic or bad ones) are in the safe range between 4-8 Ohms, some high-end speakers might go down to 2 Ohms sometimes....this was different 10 years ago when speakers like the Quadral Vulkan went down to 0.5 Ohms in some (bass) frequencies and fried the amps of a lot of people...
and right, it's a good idea (a must imo) to use at least a capacitor to filter the lower frequencies (4.7mF is pretty the default value so it should be safe to use it). if you're willing to spend a little more money for better sound you can use tinfoil-capacitors instead of a normal electrolytic one for example.