Welcome Andre
I would go with the K20d over the K10d.
You should take a look at the comparison, if you haven't already --->
Pentax Cameras | Pentax K200D vs. Pentax K20D vs. Pentax K10D - Pentax DSLR Comparison - PentaxForums.com
I think the higher price of the K20d is well justified by more MP, a higher iso range (although if you go 800 or over, things get noisy, but still an improvement over K10d), pixel mapping and auto-focus adjustment (not vitally needed, but nice bonuses to have), and option for using live view.
My advice to anyone and everyone is when buying lenses, to look around on Ebay at old Pentax film cameras and at the lens that are included with them in the sale. Many times, you will be able to find what you want for a much better. If the camera comes with an additional lens/lenses that you don't want, just sell it to earn some money back. That is how I built up my manual lens kit back when I started out with my DSLR.
A nice M42 lens set-up that won't cost you a fortune would be Takumar 28 3.5, 50 1.4 (50 1.8 is also a good option and a lot cheaper), and 135 3.5. If you want something a bit longer, 200 4 is also a good, cheap lens. Another word of advice, go for the original/genuine M42 to Pentax adapter- those cheapy Ebay ones from Asia aren't always a perfect fit so they are hard to get in and out of your camera.
A good A series set-up will most likely run you a bit more than M42 lenses, but if you could find any of the following at good prices, then you'd be good: A 28 2.8 (if you're willing to give M series lenses a try, the M 28 3.5 is also a good option), A 50 2 (you can't go wrong with it, there are tons of cheap used ones floating around, but if you want something a bit better and faster, there are the 1.4 and 1.7 to consider, and of course, there is also M series 2, 1.7, and 1.4 which are cheaper if you're willing to give up the "A" setting on the A lenses), and a zoom/s to fill the remaining gaps and fill the telephoto range A 35-70 4, 35-105 3.5, 70-210 4.