G'day HayTay,

have a look at the reviews at http://www.photozone.de/ to guide your decision making

you may have to go to canon or nikon lens mounts to find a review for some (most) of the lenses by sigma, tamron, tokina, zeiss etc (more frequently reviewed in these mounts due to sales volumes etc) and then go to the respective manufacture's site or your preferred retailer to see if they make the lens in a pentax k-mount. Noting that tokina don't make many lenses for pentax anymore but as Falcon's points out pentax and tokina do share some lenses & technology, confusing, yes! photozone is usually good at sorting this out

all pentax mounts regardless of brand will work out of the box, some of the new autofocus systems will not work with old pentax bodies for eg Sigma's rather nice HSM system does not work on older penatx dslrs but you should be ok with K-x being a new model,

Kym did I get this right?

but always check with the manufacturer specs to see what camera bodies are compatible.

cheers,

Tim