Whilst Nikon and Canon seem to be the DSLR of choice I have to pull you up on this point. Pentax have a BIG benefit for they use essentially the same lens mount from the K1000 all the way to the latest DSLR.
Therefore there is a HUGE range of lenses to choose from that all fit straight on without an adapter.
Yes thats true, but the same is true for Canon. (not a Nikon guy so can't say for sure about that.) Any EF lens will fit on the newer EF Canon mounts.
Now the down side of being able to use older lenses is that the older lenses don't have the speed or quality glass as the new lenses do. In addition alot of the older lenses, even though they will fit on the camera, do not have the electronics to utilize the Auto Focus features.
EVER ELEVATE-
Buying a new entry level DSLR is a good idea. Basic ability of the camera is good for a beginner.
More important than having a good camera and glass though, is having a base knowledge of photography theory and concepts. The best equipment in the world with no knowledge of photography principles will only result in bad pictures, and a bad experience tending to make you want to give up.
Read about exposure, lighting, composition etc. Get the basic ideas down first. Practice with the basics and realize you are not going to shoot gallery, or magazine quality photos in a few days. Continue to read and learn and apply what you are learning as you go.
If you enter this with realistic expectations and a motivated attitude you can have a long lasting and sastifying hobby and possibly career.
Good luck with your decision.