SIGMA 500/4.5 EX DG APO Macro HSM Pentax AF

Hello all,

I stumbled across a SIGMA 500/4.5 EX DG APO Macro HSM Pentax AF second hand in a camera shop at 2779,- euros.

I switched to pentax from Olympus, and still have a 150f2, which I am able to sell into the bargain. That júst about makes the sigma affordable to me. I'm wondering about some things though:

-does a HSM version of this lens actually exist in Pentax mount, or will focussing rely on the screw drive motor in the K5, and if yes: is focussing sufficient for birding?

-Is the "macro" version a lot different from the non-macro?

-how old will the "DG" version be? Because this lens in itself goes back quite a while it seems.

-Is the price realistic, and is the lens worth it?

-Are there any things to look out for when buying this lens secondhand?

thanks a lot in advance

Chris