Welcome! Yes, any Pentax lens ever made (with about 3 minuscule exceptions) will fit modern Pentax dSLRs like your K10D. M42 screwmount lenses (those made by Pentax are usually labeled Takumar) will fit, with a simple adapter -- and we have lots to say about adapters! Lenses with the PK bayonet mount need no adapters. Lenses from other makers will also fit but may need adapters. Certain lenses made for the Pentax-Ricoh KR bayonet mount, and certain lenses with the Vivitar name, have mount issues and need minor surgery to fit properly.
As Lowell said, various older lenses may have functional limitations, usually having to do with 'automatic' aperture operation. Just because a lens fit and works doesn't mean that it will be easy to use, a source of masochistic pleasure for many of us manual-lens pervs. The easiest lenses are 1) those that are autofocus (AF), and 2) those with A-type contacts on the lens base. All other lenses require a little easily-learned technique. We have great fun with this.
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No, ultra-wide lenses aren't necessarily best for shooting 'scapes. Any UWA (ultrawideangle) or FE (fisheye) lens will shrink the distant -- mountains turn into molehills, city skylines become bumps. Very short focal lengths are usually best for 1) shooting in very small spaces, or 2) shooting to emphasize something close while diminishing its background. If we look at published collections of 'scapes, we see that most of them were shot in a focal-length range equivalent to the 18-55mm kit lens. I personally prefer a 24mm lens if I can stand a little distortion, or a 28mm lens if I want the image distortion-free.
That said, UWA and FE lenses are good to have and fun to use. The lens that brought me to Pentax was the wonderful DA10-17 fisheye zoom, very fishy on the wide end (10mm), subtly fishy on the long end (17mm). Non-fishy (aka rectilinear) UWA's have a different distortion, often more pleasing to us than is curved fishiness. Baro-nite mentioned the Sigma 10-20's and Pentax 12-24. I have the Tamron 10-24, a focal-length range I find very useful for tight spaces on the wide end and 'scapes at the long end.
But how good are these lenses? The fishy DA10-17 and rectilinear DA12-24 have excellent optics and builds. With the Sigmas (and the Tamron to a lesser extent), some users report having to return lenses several times before obtaining a good copy. I have no problems with my Tamron 10-24 and I love the DA10-17; I have no experience with the others. It's a good idea to read the lens reviews here when considering an UWA or FE (or any other) lens. And be sure to buy lenses from sellers with no-questions-asked return policies.
Last edited by RioRico; 09-21-2011 at 07:19 AM.