A while back I won a Sigma 20mm f1.4, which is just perfect for my uses. Unfortunately it doesn't come in K mount. Sigma offered me instead any other lens up to the price. £600 or so. haven't taken up the offer yet and I might be able to choose a more expensive lens paying the cash difference (although I haven't asked).
Are there other lenses I should consider?
I already have the 3 Ltds and the Pentax 100 macro is good enough.
The 300mm f2.8 and 500mm f4.5 come in PK mount if you really want to push your luck
But for FF, the only knowledge I have is a friend of mine (Canon user) bought the Sigma 35mm f1.4 when he damaged his Canon 35mm f1.4L and he rates the Sigma better then the Canon, even though the Canon costs twice as much, So worth checking if that's available in PK mount?
What about the 18-35mm f1.8 if you don't need FF, it's a lens I've always fancied but could never justify?