You won't need a digital scanner for most of the normal public safety listening in King county, but you will need an 800 MHz and trunking capable scanner.
I use the BCD436HP by Uniden and would highly recommend it. It supports many forms of digital and trunking and is easy to program. And with the introduction of the SDS-line, the 436 is dropping somewhat in price. Of course, it doesn't do as well on some digital simulcast systems as the SDS-line does.