I would have to agree with Rio on the K20. I have had mine for 18 months, and it has everything I need. I too do landscapes but very little wildlife, however I do do ambient low light, at low iso speeds. I have read about the K5, wonderful camera body - however, I feel no compunction to run out and spend $1500. I have seen several factory refurbished sales of K20 units with a kit lens for $499. As previously indicated, the K20 is the largest body Pentax had made (still small compared to other brands), so I would think that it would fit your larger hands probably quite well.
Its controls and functions put it at the semi-pro level, what ever that means. There is really nothing that I have found that it can not do - and its the last Pentax body that supports tethering for some unexplained reason.
If someone gave me a K5 - I would use it, but I would feel very guilty, since the K20 has been serving my needs quite well. I can possibly see keeping it till my skill takes a leap (unlikely) or it dies (hopefully never).
If the $500 mark is a barrier (and I can understand), the K100 is a wonderful body - I still have mine, however the capabilities of the K20 are far and away better.
Hope that helped.....