Our first DSLR was a Pentax K-x that we purchased so my wife could take pictures for her online business. After playing with it a bit, I decided it was time for me to get my own DSLR. The K-5 got good reviews, so I picked one up and never looked back. It is definitely a more robust, fully-featured, and seriously-minded camera body than the K-x.
Regarding your questions:
1. The K-5 won't match a modern DSLR in terms of high ISO image quality, but it does support higher ISOs than the K-x.
2. The same goes for autofocus, thanks to its newer SAFOX system.
3. I haven't noticed any occurrences of the mirror flop issue.
4. I didn't consider the K-5 II to be enough of an upgrade from the K-5 to justify buying one less than a year after I bought my K-5, but either camera would be a significant step up from the K-x.
I should point out that the K-5 is noticeably bigger and heavier than the K-x, and its battery life is supposedly less (I wouldn't know firsthand, since I use a battery grip with my K-5 while our K-x is powered by an AC adapter).
You might find this link to be useful:
Pentax K-x vs Pentax K-5 Detailed Comparison