Originally posted by wadge22 Welcome to the forums!
That's a tough question, since it's very much about the budget. K3iii will definitely be better at AF (and in many other ways) than K70. You could also look at some intermediately priced options, like a KP or K3ii, either of which you will likely need to buy used. Any of those cameras will be better in a lot of ways than your K100D Super (my first DSLR, of which I have fond memories). The K70 is very capable, though, and a great bargain. In my mind its main drawback is the less robust build quality compared to the higher end Pentax models.
You probably should not consider the K1 or K1ii. You are better off with an APSC sensor camera for the "crop factor," which "increases the reach" of the lenses you use. Some people have problems with those terms, but the concept is sound: APSC bodies are generally a better choice for birding/wildlife.
Another important consideration is the lens you will be using, since birding really does require quite a bit of reach. The main current choices for bird/wildlife shooting on Pentax are the DA 55-300 PLM, the DA* 300, and the DFA 15-450. If you don't have one of those (or one of their older counterparts, or another good 300mm+ option), you may want to keep that in mind when budgeting. The good news is that the 55-300 PLM is a good option, a good value, and a super fast focusing lens. DA*300 is an excellent lens with terrific IQ, but is not fast to focus. The 150-450 is probably the best you can do, but expensive and large.
Bottom line, the K3iii will be quite a bit better if you can afford it, but you will also want to have your lens situation well sorted for a satisfying birding experience. Only you can decide whether the extra money is worth spending on your hobby.
Fully agreeing with above, but I will try to give some additional considerations:
K-3III and the 55-300 PLM are the latest technology in camera and lens, especially for fast autofocus. New, expensive body and probably what you like for birds in flight challenging your autofocus. Also compact!
If you need more reach , the more expensive lens pentax 150-450 lens is better and still has electronic autofocus motor but of bit older generation. Focusing will still be swift. But it is also big lens and less suited for hiking and hanging it on a camera+neckstrap for quick handheld shooting... handheld works, but carrying it is more , well, heavy.
The Da300 is probably better if you shoot in evening light, you gain a (few) stop(s).
If you have some older tele lenses, they are probably screwdrive autofocus, using the noisier and slower cameramotor for focusing. Will still work for birding focussing ( like not everyone drives porsche or ferrari’s , but other cars can be reasonably fast).
Any recent 24MP Body like K70, any K3, KP (some only secondhand) are very good options and price efficient options. They have good recent image sensors and good autofocus systems. As you come from older camera, the improvement will feel hugely pleasing. Also the more recent in camera body image stabilisation (IBIS) will improve your shooting a lot, it can compensate and allowing a few extra stops of light (3 to 5!)
They also work fine (and fast) with new lenses with electronic motor focus (Pentax SDM or PLM, Sigma HSM), so you can still benefit from a lens upgrade(later on). I moved from a K3-II with an old screwdrive 55-300 to the 150-450 and was impressed by the increased focus (&image) performance ... , but keep the old for my lightweight lens option.