I labored long and hard to purchase a Yamaha top end Oehler horn, writing numerous letters (all of which - every one, without exception - went unanswered (including one translated into Japanese, no less)), but had zero success. None of the retail shops in Germany would answer my inquiries, Yamaha here in the states was totally uncooperative (I reached them through telephone calls), and the Yamaha mothership in Japan never acknowledged my existence.
(This was in my pre-internet days, mind you - things would go a lot smoother these days with web sites and all.)
(Also, note that a good number of the clarinets in the catalog sheets you have referenced are not really Oehler instruments. Only those with a finger plate for the middle finger, right hand, are constructed according to the Oehler patent.)
The up end makers (Uebel, Wurlitzer, whoever) aren't very interested in hearing from non-Germans, if my inquiries can be taken as a guide. Then too, their "up-end" horns are produced on an artisan basis, and they apparently maintain zero "stock", producing only to fulfill orders in hand.
I ended up purchasing a Amati top end horn from Woodwind and Brasswind. It was a returned stock horn, and it arrived in a cheesy case (with a plastic handle, no less), in poor regulation and without an appropriate German style mouthpiece.
(It apparently went into the "used" box when the horn was returned, and they (WWBW) just tossed in a typical French style one when they shipped it to me. The mouthpiece sent didn't fit the socket, by the way.)
The hardest thing to deal with was the lack of regulation. The horn, as delivered, had all but one of the rings set so high that my fingers could not seal the tone holes, press down as I may. The reason for this was easy to divine - to several of the rings are attached small vent keys, sealing adjacent vents. As these have to seal regardless of ring height, the only way to lower the rings was to remove the thick, German-style leather pads from the cups, and then replace them with custom made cork pads sanded to the proper height and then glued in place. It was not fun, but we finally got the problem fixed.
The other thing about the Amati horn was that the overall "build quality" of the instrument was far less than I have become accustomed to over the years with Selmer instruments. Even though the keywork is silver plated, there are plenty of "rough edges" on the metal, and several of the fittings on the instrument (key stop posts, for example) weren't even fully attached to the instrument. (One of these fell out during the initial overhaul of the instrument, and my local technician had the devil of a time finding in on the shop floor - God knows what he would have done had he needed to get it replaced from Amati.) The bell was also poorly finished, with rough wood within same.
It was cheaper, however. I paid under $900 for an instrument that they were selling for about $1,200 new at the time (if my memory serves me well).
As for the instrument as an instrument:
• The ergonomics are horrible - thumb rest in the wrong location for anyone other than an orangutang, left hand little finger key touches either mis-located too far around the horn or you have to assemble it with the right hand finger holes out of alignment with those on the top joint.
(I do have an instrument that is worse than this, however. My Italian "Albert" system bass clarinet puts the little finger keys for both hands in what feels like the wrong place.)
• The keywork is somewhat bassoon-like as far as noise is concerned. The persistent tendency of German clarinet designs to use "clapper" keys on the lower joint, instead of relying upon the Boehm-Klose style axle keys, is one of the more puzzling things about the German clarinet. If the French - French, for God's sake - makers could figure this out in the 1800's, you would think that the German makers might catch on in the next hundred years or so, but no...
• You will need to resist the urge to play a German instrument with your current mouthpiece. First off, it may not fit - the mouthpiece socket will most likely be of a different diameter. If the horn you end up with does not have a mouthpiece, measure the socket and then purchase a Vandoren German style mouthpiece of the correct diameter.
• You will also need to purchase some reeds appropriate to the mouthpiece - Vandoren also has these. The German style mouthpieces have a tighter lay than you will be used to, so moving upward to a 4 strength (for someone used to playing a 3) will probably be appropriate. A 5 strength is not unusual when playing a German mouthpiece.
• Finally, the German style mouthpieces are grooved for a string ligature. You can use a Rovner without a problem, but I would recommend that you use the string (beeswaxed twine works just fine for this - the string and the beeswax (purchased at your local fabric store in the sewing notions department) will set you back all of three dollars) and learn why German players have retained the antique method for so long.
Would I convert to a German style clarinet for all of my playing? Most likely not. The "system" is quirky compared to the Boehm-Klose one, and things are definitely different above high C. Also, you can look far and wide and long, but you will never find a German system bass clarinet for sale in the used market.
Speaking of that, it is possible that you will find a suitable instrument on eBay's German service. Do a search for "klarinette", and sort through the listings, paying attention to terms like "Klapper", the number of which will define the number of tone holes on the instrument (not the keys). The greater the number there, the higher the overall quality of the instrument and thus a higher justifiable price. As always, the buyer should be wary - there are fraud experts in Germany as well as here.
Having said that, there are perceivable differences in tone/timbre. If you like the German sound, then you will probably enjoy what you hear coming out of the horn.
I have primarily used my Oehler for playing shows, but the changeover to the Oehler fingering system always takes me a couple of days to handle smoothly. And, bear in mind that I have been playing the Albert system on clarinets for a long, long time - you most likely have not. There is a difference - I call it the "first fingers difference" that you will have to overcome before you can handle them well.
Oddly enough, the "problems" with the long keys (the lack of the duplicated C, Eb, B and C# keys on the lower joint) are not as bad as they have been played up to be. The "patent C#" mechanism covers most of the difficulty, and those who have played saxophone (with much the same key arrangement in that regard) will find themselves wondering what all of the fuss was over.
More than anything else, you will find yourself treated as a sort of pariah amongst clarinet players. Regardless of what comes out of your horn, you will be viewed as "different" and non-conformist. Just how you will handle this remains to be seen. (I even run into these attitudes when playing my full Boehm clarinets.)