Advice on sports camera

If you appreciate the virtues of the K-20, the move to Pentax's newest technology, the K-30, makes a lot of sense.  The only downside I see is the lack of provision for an external battery grip.  If the grip is needed, you won't go wrong with the K-5, a camera I dearly love. Right now, prices are close between the 2.  If you already have Pentax lenses and accessories, stick with the brand you know and save yourself a huge outlay to switch to Canon or Nikon.

People are going to generally recommend the brand that they like and use.  In this case, compelling case can be made for all three.

To quote dpreview.com, a reputable camera gear review site ...

The K-5 has a very similar sensor inside it to the Nikon D7000's, and like the Nikon, the K-5 displays excellent image quality across almost its entire ISO span. Ultimately, we have no hesitation in recommending the Pentax K-5 to anyone - not only existing Pentax users, ... it earns a solid gold award.

K-5, when it first came out had a rather high MSRP.  Since then, the current street price of K-5 is much cheaper than both D7000 and 7D.  As for existing Pentax lenses from film era, while they can be used as with most all older K-mount lenses, not sure how useful they are in reality, especially if they are of manual focus variety and you are used to auto focus feature of the DSLR. Hence, I understand your willingness to change brands. 

Your requirements are for indoor sport photography.  That requires a sensor with excellent high ISO capabilities and camera body with fast frame rate.  All three cameras have excellent sensors for that purpose, although many would give a slight edge to Nikon and Pentax (they use same/similar SONY sourced sensors).  All have adequate to excellent FPS (frames per second), useful for sports photography: D7000 (6FPS), K-5 (7FPS), 7D (8FPS).  A third requirement for sports photography is the use of fast, constant aperture lens. All three brands have fast lenses, although Canon and Nikon have more choices with 3rd party lens makers.  In the end, if you choose to stay with Pentax, you'll save money on the body and get a capable camera in K-5 and can use the savings toward a fast lens. If you get better lenses, your current K20D will also perform better and can serve as a backup body without having to lug a whole another system.

The user interface (UI) on K-5 should be familiar to K20 users.  As for choosing Canon or Nikon, I suggest that you go to a camera shop and try both out to see how they feel and handle in your hands and try out the menu and UI layout.  The decision should really come down to how they feel in YOUR hands.  Another factor that may or may not matter to you is 7D is substantially heavier than either D7000 or K-5, which weigh about the same. Both K-5 and 7D are substantially weather sealed, better than D7000.  Pentax has more affordable choice of weather sealed lenses than the others.  Pentax was recently bought out by Ricoh, a reputable Japanese camera and imaging company.

I think K-5 will be the best. Cause you already have some Pentax lenses, so lot of money you can save without buy new lenses. And K-5 is chepaer than Canon and Nikon. ISO performance is one of the most important advantage of the K-5. In poor light it will be very useful. Canon can shoot 8 frames per second, but I think, the 7 fps of the Petnax is sufficient too. K-30 will be the best for you if you want shoot videos. 

The K-5 is known to have amazing results with high ISO, which is mandatory for sports, I use a Nikon D7000 and Pentax K-r. The Pentax generally gives me the better results but D7000 is better in low light. I would recommend either, but I would stick with Pentax.

This is a tough one, if you own a lot of lenses (or even one or 2 good ones) I would stick with the Pentax. They make a solid camera, although they are not as popular as Nikon or Canon. You might also want to look at the Pentax K-30. 

the best dx, 1/8000 - best performance iso - lens old compatible