The Forum has an article list up on the top menu bar, that contains a lot of information. Here is the link for lenses...
There is one particular article that may help you here... Its from RioRico - who use to be extremely active here. Unfortunately he finds it hard to use both a camera and a PC now because of his eyesight woes. That said, he was a wealth of information. Here is his link about buying used lenses. The link started out as a post and turned into an article. It was written from the perspective of finding a lens in a goodwill, thrift shop or craigslist. But it does give you a good sense of what questions to ask and what to look for.
Here is an article on mounts. Pentax essentially has only two mounts - the M42 screw mount used up through 1975, and the K mount - from 1975 to today, and its variants. All the variants to the K mount work on all the digital camera bodies. The second link address some of the finer points of the various lens mounts variations.
Quote: Does it make sense to purchase this lens even though it was originally bought in 2007? It has good reviews on here.
You are essentially asking about new vs used lenses. Used lenses are an excellent source - not to be overlooked - IF they are in GOOD condition. No fungus, mold, cracks, etc. The articles above goes into more details there. I have one M42 lens built in the 60's. Its excellent. I have others from the 70's, 80's and 90's - all excellent. I have purchased used, off of evilbay, craigslist, the Forum's marketplace and used camera shops. A little care, a lot of questions, and research all go a long way. Pictures also help when dealing remotely.