And on eBay a boxed FA* 300mm for £725
http://www.ebay.co.uk/itm/Pentax-FA-300-4-5-300mm-F4-5-/132099468167?hash=item1e...
(It's being sold by pengbai on here, he'll have HMRC after him!)

There's a Pentax 28-105 for sale here including lens hood which is usually missing
http://www.ebay.co.uk/itm/SMC-Pentax-FA-28-105mm-F3-2-4-5-AL-IF-Lens-/2823645528...

Seems it was withdrawn from sale yesterday morning.
The seller appears to be doing a lot of dealing for someone not listed as a business seller.

Pleasure! All sorted now I hope?

Your gallery shots suggest you don't seem to go over ISO 800 anyhow so is that really an issue?
No AA filter = sharper looking shots, by a long way, noise or no noise.
When cropping your bird shots you'll have more pixels making up the image so more definition (even before the lack of AA filter helps definition even more.) So there might technically be more noise but the other benefits can give the impression there is less noise.
Try using 800ASA film and I'm pretty sure you'll see grain! What's the difference?

Why not download the manual? Page 65
http://www.ricoh-imaging.co.jp/english/support/man-pdf/xg-1.pdf
Full manuals are often not included these days, download only!

Basically yes, but the extent of vignetting varies on the focal length. Vignetting is worse in the middle of the range but not at all too bad at 300mm.
If you shoot it on full frame you can see in the viewfinder how much vignetting there is and frame accordingly which can allow for a more usable area than crop mode (and crop the FF image to taste afterwards.)
This is a full frame image at 120mm which shows the vignetting, but used to effect.


At 300mm the vignetting is just a little bit in the corners as here (f6.3)


But just look at the centre sharpness at full aperture!
100% crop of centre:


You could almost scan the bar code!
Some 300mm lenses wouldn't get close to resolving that or the mesh in the satellite dish.

From Ricoh's website the only compatible cameras are K-70, K-1, K-3 II, K-3, K-S2, K-S1 and K-50
http://www.ricoh-imaging.co.jp/english/products/lens/k/telephoto/hdpentax-da-55-...
Updates had worked back to the K-3 by last November.

SRS also have a used DA*300 on eBay for £599

PF reviews suggest its bad, but there are some more positive ones here:
http://www.photographyreview.com/cat/lenses/35mm-zoom/sigma/28-105mm-f-2-8-4-0-a...
The consensus suggests its a bit soft wide open (nothing unusual about that) but OK stopped down. Its how it suits you that matters! I'd normally make sure it has a lens hood on it outdoors in the sunshine to help contrast.
I've not tried one myself so can only go on what others have said but it seems par for the course for budget lenses designed for film cameras in the 90s. Providing you fill the frame with the subject all should be well, but it might show issues should you need to do moderate cropping of images whereas the new Pentax DFA would win hands down should that be needed.
One aspect to consider is how it focuses, is it reasonably fast , reliable, and accurate? It is isn't it might let you down occasionally.