...I have to say that if you have trouble with the €3000 for a used P21, then I am not sure that this is the best place to invest money. There must be easier ways to improve your results than jumping into MF at great cost. The back won't be the end of it. There will be more lenses (the 80 is neat, but not the best in the lineup by far), tripods, flashes, strobes, whatever, all at great cost. Take things one step at a time. Maybe stepping up to a 50D makes more sense, given your existing equipment.
I would agree with this. €3000 is a great price for a used back. I actually think he should list that 50% higher, but I don't know the local market conditions there or the condition of the back (I assume it has no warranty remaining).
A P21+ Refurbished back with 1-year international warranty and brand new accessories has a list price of $8,990.
A P20 non-plus Refurbished back with 1-year international warranty and brand new accessories has a list price of $5,990.
As you can see there are some great deals for these backs with factory re-certification and a warranty and dealer support (very important, especially since you're new to medium format digital). However, the deals don't even come close to €3000, and as Carsten points out the digital back, body, and 80mm are just the beginning of Dante's inferno. Once you see the quality (not just resolution) of the images and the tactile shooting experience you'll desperately want other lenses, filters and other accessories.
I would stay away from medium format digital until you can budget at least twice that. Depending on your shooting style (mainly how many frames and whether you NEED instant feedback or would just like instant feedback) and proximity to a lab film may be a viable option. Or as Carsten suggests a low-to-mid tier dSLR.
Nearly everyone on this forum agrees that the look and quality of medium format is well worth the money and slower workflow when shooting things conducive to medium format (e.g. not sports), but you do need the money. dSLRs may not be the same experience or quality, but they are DARNED good and at your budget you're probably better off.
Part of my advice here comes from my experience with many first-time medium format buyers who jump onto a "deal" - generally by buying used from a private individual - without 1) the budget for the total cost 2) understanding the system they are buying 3) understanding that they will need a dealer to help them with all of their questions and an occasional problem or two. These users often get very disgruntled.
Just last month we had a gentlemen who ended up very mad at US after he bought a used back from a seller in Miami. The orientation sensor in the back was busted and the CCD had a line-error (similar to a dead pixel on a monitor); he got the back for what he assumed was a "steal" (based on looking at the price of a used back with warranty from a dealer the price he negotiated with the seller was about half). The back had no warranty, and the price of repairs to bring the back was about as much as he paid for the system. Furthermore he had absolutely no clue about software and was doing a shoot on location in South America when he started getting corrupted images. Turns out he was using a VERY early version of the software (the one that comes on the CD sold years ago) with an incompatible version of the OS. In the end he shelved the medium format back and left my office with many swear words saying that medium format was useless trash. If he had purchased the same back from us (assuming he could have made the budget) I know he would have had a positive experience because he would have done training with me in advance of his first shoot and I would have made sure he wasn't using such deprecated software/OS combination. That sort of thing personally pains me.
By the way this is NOT a digg at footoograaf with whom I have no experience positive or negative.