Using K-mountmanual lenses will not do anything to your electronics, and the owners manual tells you how to use them.
Half pressing the shutter will show you whether you have it in focus or not (the green light comes on when the camera thinks you are in-focus, but can't focus for you), but the AE-L button is what temporarily stops down the lens and sets the shutter speed. The lens will stop down again when you take the picture and open up immediately - you don't have to unlock the exposure. However, until you press the AE-L button again the shutter speed stays the same (can't tell you how often I take a picture of something that's dark, then overexpose the next picture because I forget to meter again). I think I always half-press first because I usually focus first, then meter last. Knowing me (always doing things backwards), it's probably better to meter first then focus. Does that make some sense?
While I really like the DA 50-200 lens, I have more K-mount manual lenses than anything else, and use them regularly.