To a first approximation, the accuracy of a counter is really a function of the timebase circuitry. That's why you should ideally look for a counter with an
ovenized oscillator. Almost all counter manufacturers either include the ovenized oscillator, or offer it as an option. Having said that, there were (and are)
many different schemes used for the actual counting - the cheaper counters simply count the number of timebase clocks; the better counters use some
sort of interpolation. There are lots of descriptions of the various techniques on the web - several of the older HP Journals describe them.
As far as frequency coverage goes, obviously the higher the frequency, the more potential usefulness the counter has. But sometimes, too high a frequency
range can be a missed blessing. For instance, you can't reliably use BNC cables/connectors up at 18 GHz. So unless your lab already has a lot of microwave
capable equipment and accessories, I wouldn't necessarily place a premium on the upper frequency limit of the counter.