That is viewfinder dust, no issue. Not important also. Don't really need to clean if it doesn't bother you. Sensor dust is another thing altogether because it shows up on EVERY single shot you take.
That is the focussing screen mosaic pattern. No issue. It only shows up in very bright light, right? If it bothers you can try to change your focussing screen to another one, but I think will still have that sort of thing - or something similar, and costs money.
Wide angle lenses - Pentax has a 10-20 if I'm not wrong, if you want to save some cash, Sigma has 10-20 and 12-24. Both have got quite good reviews, but are really heavy. Otherwise, the 18-55 that comes with the kit set is pretty ok for landscapes other than some very slight distortion, easily corrected in Photoshop. I have been using 18-55 for all my landscape shots.
Filters --> It depends on what sort of effect you want. If you're ok with silhouettes only, then you don't need any. CPL will help very slightly, since you are going to be pointing directly at the sun, reducing its effect. If you want details in the foreground, get a Cokin P series set or Tianya filter set.. And start grabbing those Graduated Neutral Density (GND) filters. Me, I use Cokin.