And Ffordes have a fisheye:- http://www.ffordes.com/product/18021215283651

This has just appeared on Ffordes:- http://www.ffordes.com/product/18020718030281

Sigma 12-24mm is a very worthy consideration if you can locate one.

Are you buying or building yourself? And, how much do you want to spend?

A multicore cpu, AMD or INTEL take your pick. It doesn't need to be the latest generation of cpu, a slightly older one will be better value for money and still able to run Lightroom. Going for a minumum of 16gig of ram will help your workflow as photo editing can be memory intensive especially with the bigger raw files of modern cameras. Lightroom 6 will also also make use of a graphics card where previous versions of Lightroom didn't, so factor in the cost of a dedicated graphics card if your budget allows.

An area people often overlook when building or purchasing a new PC is the screen. Considering that this is the part that you will be staring at hour after hour, it should not be an after thought or skimped on. In fact I would say a decent screen is a major priority which will enhance the user experience no end!

If you google recommendations for a photo editing PC you will find lots of links to a wealth of advice. If you are able to build your own then do so. It might not necessarily be cheaper, but at least you put in the parts of your choosing rather than accept some manufacturers generic parts and it will also give you a measure of satisfaction.

Good luck and best wishes.

I don't have what you are looking for but I have been watching on ebay a Samsung GX 10 which is the K10d equivalent, which might be worth your consideration. https://www.ebay.co.uk/itm/Samsung-GX-10-10-2MP-Digital-SLR-Camera-Kit-w-D-XENON...
Good look with your search.

Very nice but there appears to be a couple of clangers!

If widening the field a tad, consider also the Pentacon / Meyer Optik Lydith 30mm f/3.5. Okay it's not 2.8 but it might be worth a punt
and there is one here:- https://www.ebay.co.uk/itm/Pentacon-3-5-30-Lens/322856887881?ssPageName=STRK%3AM...

The close focus version is highly regarded and more sought after for amongst other reasons the fact that it does focus so very, very close. Rather than me trying to explain, have a look here for a detailed study of all the variants:- http://photografica.robinparmar.com/vivitar.html

So many to choose from so how about this one for starters. https://www.pentaxforums.com/userreviews/vivitar-28mm-f-2-8-close-focus.html

I have the PKA version but they are available in m42 mount. Very close focussing too.

Good luck with your search.

A bit more info here that you might find helpful from a very interesting and engaging photographer:-

https://youtu.be/xvD-CKPceg0