I know nothing of this particular lens, but a prime lens will normally give better quality than a zoom - although modern zooms are much improved.
The advert says the lens has some fungus in it. Even though it is cheap, personally I would avoid it. There are plenty more cheap lenses out there. e.g. I bought an M42 mount 180mm lens not long ago just out of curiosity, and it cost me just £8.94.

True, when you compare it to the field of view on a full frame, but equally you can say that the 70-300 compares to a 105-450 on full frame.
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A used Tamron 70-300 is likely to be cracking value for money and a great starter lens.
It was my first longer zoom when starting photography again with a K-m, and used it on the K-x I subsequently bought. It's not likely to be the greatest lens you will ever have, but will probably give you the best 'bang for your buck'.

It is almost certainly going to be better than your 50-200 plus a cheap teleconverter.

If you can stretch to a bit more, then a used DA-L 300 is likely to be the next step up. The plastic mount is not a problem with them, and it is optically identical to the DA 300.

A Tamron 500 mirror lens would certainly be another nice option, but perhaps a bit of a one trick kind of lens, being limited to that one focal length. It's likely a zoom might be more useful for you right now.

Sell the 50-200 and use that to help fund a Tamron 70-300 or DA-L 55-300.
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There do seem to be a number of conflicting experiences with this.

I just had a quick scan through some pictures I took at Mallory Park 12 months ago, using a K-5 and 50-135 lens. Out of the faster panning shots, probably about half were stabilised and half not (whereas my recent experience with K-5 IIs and 300 lens was none stabilised).

So there certainly do seem to be some difference between the camera body/lens combinations.

Another forum member, Karel, put me in touch with 'ExifData' and 'ExifDataGUI' for Windows, which does give the SR info - not just whether it is active or not, but whether or not it has stabilised.

It would be interesting to find out how many of your 'fast panning' shots do have the SR stabilised, and what differences there are between camera body/lens combinations.

Does the K-30 have a 'sensor shift' facility like the K-5?
Might it be related to that?

Presumably Lightroom and Elements will follow in due course, if Adobe perceive this as a successful move.

Could signal a move to Aperture in due course!

Your panning technique is obviously much smoother than mine, Chris. I shall have to keep practising.

I also note that you used a K-5. Earlier in this thread I believe you commented about apparent SR differences between the K-5 and K-5 II, and that with the K-5 II and a 300mm lens you turn SR off. Other than the autofocus differences between the 2 camera bodies, I wonder what else could be going on. Is the AF linked to SR in some way?

I think you could well be correct, Karel, that a low enough pan rate is likely to be successful. I was doing my best to rotate the whole body rather than just the camera, but most of the time the pan rate was quite high - possibly higher than needed for the stock cars.

Thanks to Smeggy, who has now given me the answer to this
(page 122 of the K-5 II manual)
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That's the answer then. Thanks for pointing it out, Smeggy. No need to test it out any more now

Just goes to show that I should have read the manual in a bit more detail, instead of just looking stuff up!

(p.s. page 122 of the K-5 II manual, for any of you looking in that one)
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Having now gone through all the pictures I took yesterday with SR turned on, all those of static or (relatively) slow moving subjects had SR stabilised.

Without exception, every shot of fast moving and closer subjects (probably between 50 & 100) were not stabilised.

So it seems as though SR with a long lens and fast moving subjects doesn't really work (for me at least!), and I suspect some of the unsatisfactory shots I'm getting are down to sensor movement while it's trying to stabilise.

I'd like to hear any advice you can give, either with SR or how to pan more successfully.