I have an Albinar tube set that was bought in a second hand shop. It's price was 9000 HUF (about 30€). Each of the tubes has K mount and works fine.

I use external flashes. Long before I used a cheap Soligor DG-20 AS flash on bracket, now I use a Metz MB-15 MS-1 ringflash. It's price is somewhere between the DG-20 AS and the Pentax, Sigma and other ringflashes.
You can use the built-in flash too, Charlotte uses it with Pringles tube as diffuser and she takes great shots with it.
Yes, you can use the Raynox DCR-250 with your lens, it has a universal fitting capability.
What you mean under "awful", could you give details?

I agree with Peter, the last two are the best. The 3rd one would be nice if the centre of the flower would be sharper.
The flash can be useful. You can use narrower aperture and higher shutter speed with it. If you would like to take more abstract shots then you should buy a macro close-up lens (Raynox DCR-250, Soligor +5Dpt) to increase the magnification ratio and decrease the DOF.
The extension tube set is a useful accessory, too. It increases the magnification ratio and the DOF isn't reduced as strongly as the close-up lenses do it.

In my opinion the tube-set is a better choice than the Raynox DCR-250. The DCR-250 vignettes on some lenses and makes narrower DOF.

The principle and the effect of them is same, the difference is in the usage of them.
Using the bellows you can increase the picture distance (between the lens and the sensor) stepless but it is vulnerable and isn't too practical on the site. The tube-set is well usable but you can increase the picture distance in steps.
My tube-set contains 12, 20, 36mm tubes. As I measured it the magnification ratio is 2:1 if I use my Sigma EX 105 DG (1:1) lens and the 68mm tube-set. Using a bellows you can reach higher distances and magnification ratio.
The issue of DOF. By my experience the DOF is about 8-10mm using 68mm tube-set, Sigma EX 105 DG lens with aperture 32 at 1:1 magnification ratio on the lens.
Metering. I have manual tube-set, without electronics and I use my camera in M mode. I also use a Metz MB 15MS-1 ringflash. I can generally use ISO200, aperture 32 and 1/125 shutter speed.
The LED camera light that you linked in is interesting but I think that is unusable. The distance between the subject and the front lens is too small, from bracket you can not light in your subject, your lens will make shadow. I tried to use a Soligor LED ring light but that didn't give enough light. The cheapest (brand new) ringflash is the Metz MB 15MS-1 on the market.
Or you can ask Charlotte here int he PU about the lighting with Pringles tube. She makes it well.

Do they therefore give the exclusive discount?

Some days ago the PU logo disappeared below my avatar and I can upload only one picture per day again. My profile information tells that my membership is valid until 8 July 2011.
Could anybody help me?

Also you can try the Zoner Photo Studio. I read about it in another topic. I didn't tried it out, but the the screenshots look good.
It has free and trial versions too.

There are many other products that you can use. Adobe isn't the only one on the market. You have to decide what you want: a pixel graphic program or a photohandling software. The Photoshop and Photoshop Elements is a pixelgraphic program. The Lightroom is a photohandling software.

Some other possible photo handling SW:
A good alternative would be the Bibble5. It has Pro and Light version. It works on Windows, Mac OS X and Linux and it has free trial versions.
A free alternative is the RawTherapee. It works on Windows and Linux and totally free.

Both SW use non-destructive editing tools. It means that it keeps the original (generally RAW) files untouched and every changes are stored in a separated file. When you look at a modified picture then it repeats every changes again on the original RAW file.

My suggestion is that try them out before you make your decision.

I use A5 sized Bamboo tablet that is similar to Intuos4 and I love it. It improves my productivity in the PP. I can do more easier the cropping, the retouche, the cloning, etc processes. The pen doesn't need any battery not like the products of the other manufacturers.