Lens changes don't necessarily have to cause dust problems. There's a trick to it which the manual doesn't include:

Find reasonably dust-free area
Turn camera off
Hold camera facing downwards
Have new lens ready with rear cap removed
Rapidly unclip lens and fit new lens
Replace rear cap on other lens

I do it this way and seldom have any dust, at least none that I've noticed in photos or that the dust alert thing has picked up. You might also want to set the camera so that it cleans the sensor every time you switch on, I've done this and suspect it helps.

Shooting infra-red has made it clear why the shutter speeds are as they are on my KX. I hadn't previously realised that each click of the dial equates to a stop until I found that in order to apply the required seven stops of compensation for the IR filter I had to turn the shutter speed down seven notches...

I have a Sunny 16 card and keep meaning to try to memorise it, just so I know whether the metering is being vaguely sensible.

You're likely to end up spending big money on any 300mm prime to be honest - I've seen the K 300mm f4 go for £150 or so on a couple of occasions. Very few people bought lenses that long on film, and even fewer bought primes (the standard telephoto was a 135mm prime, then the 80-200mm zoom took over). As a result they're rare and sought after.

If you can stand dropping to 200mm then the M 200mm f4 is an excellent lens as is the A series version although the latter is hard to find. SMC Pentax A 70-210mm is also f4 and apparently loses nothing to the prime at the long end, so may also be worth considering.

That's the thing about selling to a dealer - they need to make their mark-up so won't be able to offer you the full value of the item.

If you listed it on ebay for £100 you'd probably get your hand bitten off within hours, I saw a K 35/3.5 go for about that much in an auction recently in similar condition to yours. As you say though, the K lenses are just too nice to sell! I now keep my eyes open for more of them even though I have everything from 18-300mm covered with various lenses, they're just superb.

The only one I could suggest would be the Samyang 8mm fisheye - about £220 or so last time I looked.

Remember that the 28mm was a wide angle lens on film but not on digital, thanks to the crop factor. It does make an excellent standard lens though.

If you can afford it and are willing to pay the price then go for it - I doubt it'll lose any value and it'll be a great lens. Personally I'd be more interested in putting that sort of money towards a DA Ltd, my top price for a manual prime is about the £100 mark (and for that it has to be a bit rare!)

I'm not sure whether the K-m can do infra-red? I did briefly try the filter but just got blackness even after adjusting the exposure settings.

I've been using Ilford SFX in my KX, along with a tripod and cable release. Takes about 9mins to develop then another 5mins or so in the fixer.

That is useful - I was wondering how many stops a given ND filter takes off. I have an ND2 and ND4 but haven't used them much.

I may try adding an ND8 to the collection, stacking them, and doing some interesting things with lakes. I've been getting the milky water effect with infra-red film recently (where shutter speeds of several seconds are common) and it'd be interesting to get it in colour.

The focus confirmation should work with all lenses, to give an idea of which part of the system may be at fault. I know the one I have is scruffy but basically fully functional (if I see a tidy jammed one it will become a tidy fully functional example after a bit of baseplate and focusing screen swapping!)

I would rather see a return of stereoscopic photos than more of the current 3D. At least you can look at one side of the stereoscopes without the glasses and not end up with a headache!

I have seen suggestions of 3D displays which don't need glasses, presumably using a similar technique to some car dashboard screens which display a satnav to the driver and a DVD to the passenger.