If you're after a quick diffuser, Cigarette papers also work very well, you don't even need the rubber band & they're smaller in you camera bag.
Unused tissues/toilet paper will also work but aren't so convenient.

As a student my Dad had given me a ME, and I'd since built up a small collection of lenses, (& a P30T). So continuing with Pentax seemed logical, when I was finally after a DSLR.
I'd stopped using the Pentaxes when I went digital with a series of compacts, but their limitations almost stopped me doing photography altogether.
When I heard of the K100 it seemed to offer all I wanted from a camera, but the price was higher than I could justify
Eventually I saw one on e-bay for £150 & jumped at it!
It exceeded my expectations getting me back into photography, and expanding my lens collection.
Having joined a local club, I found the controls on members CaNikons very unfriendly in comparison, and only the most expensive models offered any discernible IQ advantage.
When the time came to upgrade I wanted a K5 but the K7 was £200 cheaper and the only real difference IMO was the ISO capability.
I regret that £200 regularly.

In terms of finacial outlay, for quality gained it has to be the Bigmos, as I got mine new for £7. SRS' offer came just at the right time for my 25 year service award - but I had to pay the postage as it had to be ordered by work. Including the 25 years 'hard labour' into the equation, probably changes thing somewhat.

Other contenders are the Sigma 24mm f1.8, DA55-300 & DAL 35 - all of which I've brought new for around half the normal retail price - and the Cosina 100mm macro, which cost me £50.

In it's way my 1930's Kodak 12cm Astigmat (under £10 & used with a home made adapter on bellows) deserves a mention too. I've got both older & cheaper lenses, but they don't get used as much!

Yes they work OK, but for wider angle tilt/shift (certainly anything less than 50mm focal length) the lens ends up very close to the body, limiting movement. Retro focus designs used in SLR wide angles are probably much more user friendly.

Unfortunatly when I made my plunger mount for enlarger lenses, I made it so the 50mm focuses to infinity easily (limited movement) and then found for the 75mm I need an extension tube to reach infinity focus. I must find the time to make a longer one!

Then I might be able to get the sort of effects jkdlee has displayed... (Wishfull thinking I suspect)

I've had a couple of the cheaper ones, which have failed due to the buttons being pushed in too hard whilst being carried around in my over-loaded camera bag.
I've recently ordered replacements a super budget £2 model one with a built in intervalometer. This time I intend to find rigid box for the one I carry, and eave the advance model in a draw for when I need its special features.
I've not seen any that look substantially more robust except for DIY models (they are very easy to make)
BTW many (but not all) Canon models use the same connector

Whilst they are somewhat better than the £5 chinese ones I got, they don't have any advantage over my old Jessops ones (I actually prefer the lens release controls on the Jessops model!)

As they don't even have the electrical contacts £80 would be overpriced IMO, but perhaps there are collectors who might disagree with me on that.

I've got one, and as others have said it works reasonably for macro. Despite the focusing scale suggesting it will work at infinity, mine is soft enough at long distances that focus confim will not ever light, and even my eyesight has to agree!

Results with my mirror lenses have certainly proved better for distant work. I've had both 500mm & 600mm mirror lenses, while neither were a patch on the Bigmos (Sigma 150-500) they cost less than the TC your after, and I suspect are about the only reasonable way you'll get beyond 320mm without paying a fortune...

Bill you may be surprised how cheaply you can get EQ mounts.
I got a 113mm Newtonian, motorised EQ2 mount & a few extras for £150.
If your patient looking at local used adds (e-bay or classifieds) you can end up with some real bargains, mainly because the weight often limit sales to local collection.
I've not used mine much (the weight factor again) so have yet to really master aligning it, coupling the camera to the scope (direct conection will not focus anything near infinity)... but the mount should work fine for piggy back mounting, and with a bit of work projection through an eyepiece can work - I've got images of Jupiter's moons.

I think Peter overstates the Ricoh pin, I'd expect it to be far less than 10% that have it so 'some' rather than 'many'. It's fairly easy to remove but I guess important to check on.

As well as the M42 lenses, T2 mount & adaptall lenses can also be used with readily available adapters (many of the adaptall ones work with auto aperture if you get the PKA adapter.

Personally I prefer 2 & 3, but as Algi says all are good. Mind you we'd expect nothing less given your past record!