Hi,
Choosing speakers is always very subjective.
I would be wary of the Petit Orgues for a number of reasons. Firstly, my choice would not be for a kit. I am not sure that my woodworking skills would be adequate. It also means that you need to add on the price of the wood to the cost of the kit and if you can't make it yourself then there will be more costs.
Secondly, the efficiency is not good if this site is to be believed:
http://www.hifisound.de/en/Do-it-yourself-Products/speaker-kits/Subwoofer/VISATON-PETIT-ORGUE-SPEAKER-KIT.html The quoted 76dB sounds low. Figures between 90 and 110 dB are more common. However, I am not sure that the 2.83Volts equates to 1Watt unless they are quoting peak voltage and not rms. This implies that a large amplifier will be required.
Thirdly, one report I read said that for optimum usage they need to be placed at least 1m and preferably 2m from a wall. My music room is not that big; 2m from one wall very nearly is the opposite wall!
Fourthly, they are being a bit creative with the low frequency response. They claim 16Hz but the frequency chart they show has a half power point at about 24 or 25Hz. Having said that, 25Hz is quite a good low frequency point. Many will argue that you really need 16Hz but the room you put it in will affect the response and your ears will "fill in" fundamentals automatically. They just won't flap the carpet for you.
I already had my speakers before I had Hauptwerk. These, supposedly, went down to 45Hz and I found them quite good enough for piano sounds (30Hz min) and adequate for organ. Eventually I bough a subwoofer and that created problems because I had no idea just how many things in the house vibrated at those low frequencies! I spent some time trying to damp out those vibrations.
My personal choice would be to spend my entire budget on a pair of powered or active monitor speakers with the lowest low frequency figure I could find consistent with the upper range being accurate. Later, if I still missed the low frequencies I would look at adding a subwoofer. Most of the sound information is in those higher frequencies anyway even for a bass note. So if you get a poor set of main speakers it will affect everything.
The way I would pick them is to Google something like:
budget monitor speakers
then when you find a promising one use Google again to find some reviews. For instance, Google:
Adam F5 review
to see how the F5 compares with other similar speakers.
I am sorry that this is a bit subjective. I suspect that everyone will have their own method of choosing. Just remember that you will need speakers and amplifiers either as separate units or as a combined active/powered speaker so you need to price them both together. Cables can be expensive too.
I hope this helps,