I used to have two Canon fit Sigma 500 DG Super

To clear this up, the LCD and a couple of funky features are the only difference between ST and DG Super - they should both do PTTL.

However, according to Sigma they may not be compatible with Pentax flashes in wireless mode. In Canon style they seem to use the Canon wireless protocol - but they say you need a Sigma flash to trigger them on Pentax which suggests they don't do the Pentax wireless protocol.

If you don't need the wireless stuff, the Sigma flashes are powerful and easy to use - and cheap.

I bought a 540FGZ on Ebay brand new from the UK for £200. I got lucky - I'd like to see anyone else pull that off in a hurry!