I had K-x, and as you said, it's a great camera for learning about digital photography. Sold it 4 weeks ago, and I really miss it.
Just ordered K-5 IIs from B&H, since I believe that would be a great upgrade over K-x. Don't need high pixel count of K-3, and I'm also not doing any sports photography that would benefit from the improved AF tracking.
I really dig AF in low light, though. -3 EV dependable auto focus sounds really good.
Not to mention that K-5 IIs is just about the best deal in history at this moment.
Can't wait to see how it get's along with Tamron 17-50 2.8, which was quite sharp with K-x (once I dialed in the front/back focus correction ). I also have a Pentax A50 1.7 , and I'm going to use it with a reverse ring for some macro action
Had some experience with mirrorless but not with A7. It was a Nex 5. Generally, I hated the interface, which felt awkward compared to a "real" DSLR. That said, I'm sure A7 has a much better interface.