I have a similar dilemma. A new PLM 55-300mm arrived this week and first experiments show me that the images and handling are really good.
See in the similar thread
55-300 PLM and... Help me spend my money?!? - PentaxForums.com where I listed all the lenses that are covered by the new purchase, and received useful advice from members. I will be selling a couple of lenses that have been favourites in the past but now are out-performed by the new lens.
I have an Adaptall SP 60-300mm and that is the only older lens that I will certainly keep and use. It has great IQ, even wide open and also with the 01F Adaptall teleconverter. Its also a very convincing piece of classical photographic engineering that looks great, feels solid and functions perfectly
I suggest that the SP 60-300mm +01F combination would be ideal to keep for moon shots. You will be using a tripod, bench support or similar, so the weight is actually a stabilizing factor (reducing physical movement of the whole setup when you are outdoors) and no filters on the front. The moon images that I get are really sharp and colours are impressive, and manual focus is not an issue with the moon -it's not going anywhere quickly.