Hi Lydia, Yes, any K-Mount should fit and the numbers you are referring to are probably the range of the lens, or the aperture opening. Both of these numbers are important. For example I use a 50-200mm f4.0-5.6 . The first set of numbers is the range. In this case the 50 mm is approximately what the eye sees, as explained to me, and it will zoom approximately 4 times to 200mm to bring your view in closer.
The 2nd set of numbers is the size of the aperture (opening) on the lens. This is the amount of light that the lens will allow in.
I am not really the one to help you choose the perfect lense, however, as I mentioned above, this is an excellent beginning lense as it offers enough close in range and yet gives me the zoom I need to get some nice close shots from a distance. With 300 pounds to spend you might consider an 18-50mm lens for landscapes along with a nice zoom. The smaller the number the wider area your lens can cover. The larger the number, the closer it can zoom in.
If you are narrowing it down, go ahead and list your choices on here and we can advise you. Also, check our reviews section for the lenses you are looking at. You can find the link at the bottom of my signature where it says "Your reviews".
Hope this helps a bit.