Saulius: I strongly urge you to post a message on the dpreview Pentax SLR forum
for more information on Pentax lenses. Or just search and browse there because this topic is constantly being discussed - Pentax owners are passionate about their glass. I'm no expert but there seems to be a lot of misinformation and out-dated information on this subject from people who don't know Pentax that well.
To me, the real question, especially since you are on a strict budget, is not how many lenses a given company has in production but how many of those will be useful to you. All dSLR companies are issuing new "D" lenses that are optimized for their APS-imager digital cameras and Pentax is no exception.
Yet another consideration: the K10D with its 10 mp imager really needs the very highest resolution lenses to take advantage of the extra resolution the imager offers. For example, it is being sold as a package here in Canada together with the same 18-55 kit lens that was first offered with the 6 mp *Ist models. I have this lens and it is very nice, but to take advantage of the K10D's extra megapixels I would have to upgrade to the 16-45 or any of Pentax's superb primes.
I experiment a little with art fotography, but my favorite is landscape photography
Landscape photography can normally be done with a tripod, so image stabilzation isn't absolutely necessary; but IS is a wonderful feature that does free you to use the camera in many more situations. The built-in IS of the Pentax K line together with the backward compatibility of the lens mount means you can buy nearly any Pentax lens ever made, including wonderful old second hand primes, and still have IS enabled.
It would be very hard to help you choose between the K100D and the K10D, let alone the Alpha, which I know nothing about. I have the 6 mp Pentax DS plus the 18-55 lens and I can tell you first hand that with careful technique you can do excellent landscape prints up to 20x30cm and often up to 30x40cm. If you have a strong interest in making larger prints of highly detailed subject matter, then you would have to spend the extra money on the K10D and on more expensive lenses. On the other hand, you may feel like I do that a good view finder is very important. If I'm not mistaken the K100D does not have the excellent 0.94x pentaprism that is in the Ist D, the Ist DS, and the K10D, so buying a second-hand Pentax D or DS might be worth considering.
For some reason Nikon D80 is very expensive camera in our market.
My understanding is that in each market there is a child company for a given manufacturer or an authorized distributor that sets the prices in that region. For example, in the US Mamiyas were significantly more expensive than they were in England, probably because the Mamiya distributor in the US wanted to make a large profit on a few sales rather than a smaller profit on more sales. Another example: I've read that in Australia Pentax has a large share of the dSLR market because the Pentax distributor there is smart enough to price competitively.