I don't know if this topic is still hot. However, I am a beginner and found a grower in Germany who is specialised on outdoor cacti growing here.
Homepage (in German): http://winter-kaktus.de
I translated the most important facts:
The following cacti can be grown outdoors in Germany (up to -20° Celsius, some of them up to -30° Celsius; 500-700ml rainfall, no rainfall protection):
- Echinocereus triglochidiatus − coccineus group: (monacanthus, gonacanthus); mojavensis, v. inermis; coccineus, v. paucispinus, v. „minor“.
- Echinocereus caespitosus − baileyi group: „reichenbachii“, caespitosus, perbellus, baileyi, v. albispinus
- Echinocereus engelmannii − fendleri group: some fendleri-forms and engelmannii v. variegatus
- Echinocereus x-roetteri, x-lloydii group: x-roetteri, x-lloydii, x-rosei, x-guerneyi, x-octacanthus
- Escobaria vivipara group: vivipara, neomexicana, arizonica, kaibabensis, radiosa
- Escobaria sneedii − orcuttii group: sneedii, leei, orcuttii, v. königii, v. macraxima, villardii, guadalupensis, sandbergi
- Escobaria missouriensis group: missouriensis, ssp. caespitosa
- Pediocactus simpsonii (note that they might need rain shelter)
- Platyopuntia with big flat branches: engelmannii, phaeacantha, cymochila, compressa, humifusa, erinacea, macrorhiza
- Opuntia fragilis group: most of them
- Cylindropuntia imbricata
- Maihuenia: poeppigii, valentinii, patagonica (note that not all clones are hardy)
I took this information from here: http://winter-kaktus.de/index.php?option=com_content&task=view&id=39&Itemid=108
There you can also find some cacti which are less hardy (up to -15° Celsius) at the end of the page.
Note that I did not include the hybrids.
Please excuse me if I didn't write the names properly since I am a beginner and just tried to translate the info from the URL above
But maybe someone might find this useful.
I started with a Cylindropuntia imbricata, an Echinocereus cocchineus and some succulents. Let's see if it works...