It sounds like Robert may be in a position to help you through the DIY process. DIY is an attractive prospect, however, beginners sometimes run into problems, enough so that they abandon trying to finish the subwoofer. For some people not DIY is a matter of their time being more valuable than trying to save money on a DIY project. Others don't have the tools or willingness to take the risk of a DIY project. I couldn't give you the slightest idea what the ratio of DIY to buying finished commercial subs is. Having said all that, your location in Europe limits your access to many of the best internet direct subwoofers. If you could get an SVS PB-12 NSD for 600 Euros, you would be in pretty good shape. But since you have been exploring subsonic frequencies, DIY is by far the least expensive way to achieve >100 db at the listening position at frequencies under 15 Hz. One last thing, I would suggest that you change your plans from a sub with a passive radiator to one that is sealed. You might have to build the box a little bigger, but there are advantages to sealed vs. passive radiator designs, if I am not mistaken.