Hi Chris.
There is already plenty of good advice in this thread. I might just go back a step, as AdrianM has done.
The old lenses you are referring to were designed for 35mm film cameras. 28mm was a wide angle lens on a film camera; a standard view lens was 50mm. On your K-50, 28mm is equivalent to about 42mm on a film camera, which is just a little wider than standard. That is because the sensor on your K-50 (and all other Pentax DSLRs) is about two-thirds the size of a 35mm film frame. To get a lens on the K-50 with an angle of view similar to that of a 28mm lens on a film camera you would need to go to about 19mm.
Lenses wider than 28mm were not terribly common on film cameras, and the good ones now command premium prices. For example in the Pentax-A series (auto aperture, manual focus) the 15mm f3.5 and the 20mm f2.8 primes are rare and expensive. By contrast, prime lenses that were common on film cameras, like 35mm, 50mm, 85mm and 135mm, are now often good value as standard-telephoto lenses for DSLRs.
So the upshot is that old lenses are not a cheap route to wide angles on a DSLR.
Your 18-55mm kit lens gives a reasonable wide-angle range. The highly-rated DA 16-45mm goes a little wider - the extra 2mm makes a difference. But although not expensive that lens will be out of your budget.
Most people who want to go significantly wider than 18mm on a Pentax DSLR go for either the DA 15mm that AdrianM mentioned, or one of the ultra-wide zoom from Sigma, Tamron or Pentax (e.g. 10-20mm or 12-24mm). There is a very good comparative review of the leading zooms in this range on this site. None of them is really cheap. Better to work with your existing lenses first and get familiar with the new camera.
I'd say follow the advice above: go with the 18-55mm, maybe get a tripod (borrow a good one for the trip rather than buy a cheapie), and try stitching software to create panoramas. The free program Autostitch (
http://www.cs.bath.ac.uk/brown/autostitch/autostitch.html) is amazing.
The learning curve from P&S to DSLR can be a lot of fun. You will be amazed at what your camera and lenses can do. Read the manual, read the articles on the forum and experiment.
After a while you will see which focal lengths you use the most, whether you shoot mostly in good light or not, and so on. Then you will be better placed to look at the best upgrades from your existing lenses.