I found this site here, which about half way down the page has 2 types of gauges for sale. But when trying to buy them, you have to buy all different sorts of circuitry and make it yourself. Which is a problem...

Hardware is really not my forte. I most definitely wouldn't be able to assemble something like that, I don't even know what I'd have to choose from the list What I'm looking for, is somewhere that's selling a ready-made, assembled USB/LPT instrument cluster, preferably with a SDK included, for ~€50, possibly a bit more. But my searches have been fruitless

Is there any such shop around that anyone's seen, or would anybody even be able to assemble an instrument cluster and sell it to me?

Thanks

phidgets are probably the closest you can get to that..

Ah, much better Haven't looked at these before.

Would this be what is needed for a 4-gauge cluster?

Well you ain't gonna find gauges + electronics that can drive it for 50 euro's. Those prices go in the hundreds really fast if you can even find those. That phidget thingy might do the trick, but you will still need the gauges which are the most expensive part. You'll also need a way to connect them mechanically to the gauges if that is possible.
Also keep in mind that analog gauges can't be changed on the fly. That might give a problem if you like racing a UF1 and an hour later a BF1.

Though you could go and try looking at flight sim forums like frugalsworld, FS series or Falcon 4.0. There are some fanatics at these sites that build whole plane cockpits in their living rooms. This might inspire you or give you some information about where to find parts.

I don't think that servo kit is a good idea. Servos would give you gauges that move in the same way the seconds hand moves on a clock, ie. servos have a certain number of predefined positions, not an infinite number of positions as a real gauge would have.

Well that kit is supposed to be able to move to within 0.2 degrees of where you want, so I presume it moves in steps of 0.2 too? That shouldn't be noticeable to the naked eye, I think

I didn't think this was going to be so damn expensive...if I could even get the kit for ~€50 (I'd stretch to assembling something like the phidgets kit, but I wouldn't attempt to put something more complex together...), I could probably make the actual gauge part myself

Well then why not start with something small like a gear indicator, rev bar. If you have a feeling for programming then you are halfway there.
Get a microcontroller like a PIC which has depending on the model only 35 ~ 70 assembler instructions and wire it up. You can also program them with BASIC, C++ etc...
There are even models that have USB controllers built in, so its mostly a question of the PIC+support electronics and what you want to control.
Lots of examples are available with complex schematics. But don't be impressed, it's mostly a power supply 5 volts with a few supporting components(you could even use your PC's PSU 5V so you won't have to do anything about it). A few filters here and there for stability(you can mostly follow some guidelines) and an oscillator circuit(impressive name for something built out of 2-4 components to get a clock signal. A PIC can also generate its own clocksignal).
Price depends a bit per PIC, in the range of €2 to €20.

Well a much easier way than fiddeling with a pic is using a standard usb controller chip. It provides all you need.
I startet building my own gauge some time ago.
I used IOWarrior 24 as base and connected almost straight 16 led to the outputs. Description is here : http://www.code-mercenaries.com/




The chip comes with a small dll, and function description so it is fairly easy to programm.
Next step for me would to attach 8x8 matrix to it and attach 64 leds to it.
Sadely I didn't found suitable leds driver chips so far.

Here is a small movie showing the prototype in action.
http://www.brunsware.de/yankman/USB_GAUGE.wmv