Hi QC, to look at your options individually;
55-33 is a fine lens in whatever dress it wears (plastic or metal mount) IQ is very good particularly up to 250mm however, focus speed is slow, the full metal mount version scores here with quickshift, but still not ideal.

DA*300mm, a superb lens, but the lack of zoom could be an issue, if this is not a deal breaker then go for it.

Sigma 150-500, a fantastic lens with usefull zoom and a great OS, focussing is generally reliable, but can hunt in low light. This is my choice.

Bye the way a BIGMA is the 50-500, I may be wrong but I do not think this comes with HSM and OS, like the 150-500.

Davex.

Nice image, with good C&C from Gareth.
My in-laws live in this valley, I have never made a decent image there yet.

Davex.

Took me a while to see the girl, think you need some way of emphasizing her pose with the statue.
Not sure how though.

Davex.

Ah, I remember the day I became a Pro, Stars falling from the sky, plaudits from fellow Pros, the instant recognition of an "old hand". Pity my photos didn`t improve.

Davex.

Chester zoo today;


500mm handheld.

By posting images on here, and accepting negative criticism I beleive my photography has improved. Perhaps you need to do the same "PentaxExpert"; strange name given the lack of expertise of some the above images.

Davex.

Mini world, brilliant would look good on my wall
Please explain the method used, also note your interesting exif, is this a feature of K5?

Davex.

Have a look HERE

Have one in my K20, works fine, but seems no faster to write then my Samsung class6s.

Davex.

Am with the majority here, at least they didn`t hold onto your cash for 3 weeks, waiting to get stock. I ordered a new lens off srs 2 weeks ago, time from placing order to receiving lens was less than 24 hours .
Not many places can beat that.

Davex.