Hey there, welcome!
All K-mount lenses will work on modern Pentax DSLRs. Of course, they might not have all the features, like AF or automatic aperture.
Here is a thread on how to use manual lenses on modern DSLRs, be sure to take a look at it later, after you have a digital camera
$800 is a great budget, but it falls right between the two tiers. K-30 is the upper mid-tier camera that was released some time ago. Then there is the K-50, which is essentially the same as the K-30, with very few differences, mostly in looks. The K-500 is a stripped-down variant of the K-50. With your budget, you dont have to consider the K-500. Then there is the higher tier: K-5 and K-5II. Again, the K-5 was released some time ago and is well-loved. The K-5II has a couple more updates and features, but is very similar, almost the same camera (its main advantage is the improved AF system). The K-5 and K-5II are probably just out of your budget, so the K-30 or K-50 should be just right for you. Simply choose the one that appeals to you more. Its also great if you can hold it in your hands and compare it to other brand's cameras. Pentax cameras tend to feel much more solid than what Canon and Nikon offer for the same price.
And you can spend the rest of the budget on a really nice lens.
Pentax right now isn't making a total beginner's camera, like some other brands offer, but you probably don't need that, since you already have experience with cameras. With a K-50, you should be able to jump right in where you left off, it will have all the familiar shooting modes, with some extra features. And everything that the digital revolution brought.