So having bought a chromatic bandoneon a couple months ago, and having investigated some of these issues, I may be able to answer some of these questions. This forum has been helpful to me so here's my attempt at payback
The standard bandoneon layout for Argentine Tango is the 142 voice "Rheinische" system. Also known as the Argentine System.
There are 71 keys. 38 on the treble (right) side and 33 on the bass (left) side. Each key activates different note (voice) in the open and close bellows direction. Thus the 142 "voices".
Each voice has a M reed (at notated pitch) and an H reed (one octave above notated). Accordion people seem to believe bandoneons are LM, because the "Bandoneon" switch on accordions is LM, but that is not true, it is MH on an Argentine bandoneon.
There are "Rheinishe" system bandos that have 100,110,130 voices, and these are bandos with keys that are subsets of the 142 voice Rheinische. Some teachers write method books for these smaller bandos because they are significantly cheaper and have the same core keys. Some of these smaller bandos do not even have middle C on the right side.
I am assuming for the rest of my post that middle C = C4
There is a 152 Rheinishe that has extra keys on the left and right. Most notably it adds G3 & G#3 on the right side, so the bando can play anything in the violin part. It also fills in some holes in the key map on the left side. However, as far as I can tell, for traditional Tango music 152 Rheinishe is not necessary, 142 is the standard.
Because the price of 142 bandos have gone up recently, you often see people buying 144 bandos instead since they are much more reasonable. The key layout is very similar on the treble side, but much different on the bass side. Generally 144 bandos are MMH, though people buy these, disable the musette M reed, and use them for Tango music. This can be an option if you are on a budget, but the key mapping on the left is somewhat different.
The 142 bando is bisonoric, different key mapping for left and right, different key mapping for open or close of the bellows. So 4 different mappings of keys that you have to remember. So kinda like a keyboard that's QWERTY that changes to DVORAK when you hit the shift key, then changes to something else when you hit control- or alt-.
So in 1926 Charles Peguri came up with a key mapping for the 142 bando that was unisonoric and followed straight 3 row C system for the first 3 rows, with C system wrap around up and down for the other 3 rows. I believe this key mapping has those 3 rows go from E4 to D#6, with lower or higher keys moved to rows 4-6. The reason I say "I believe" is that I've never gotten my hands on such a bando. I've only seem pictures. Theoretically there should be pre-WWII bandos with this mapping, but all the unisonoric bandos I've found are post WWII. There isn't even a 142 Peguri option on bandochords.de . There is only a 146 peguri option. The idea behind the original peguri system was that you just change the reed plates and you get a unisonoric bandoneon with all other parts being the same. The main draw back of the 142 Peguri is that you lose A3 and A3# on the bottom end of the treble range.
The 146 Peguri adds 2 extra keys on the treble side to extend the C system range of the bottom 3 rows to go from D4 to D#6, it also restores the missing A3 and A3#. For 146 peguri 4 of the 6 reed plates on the treble side need to have extra reeds.
I also considered getting the Geuns hybrid bandoneon, but I was told from a very proficient proficient player that if my goal was to play tango music, the Geuns would not allow me proper bellows technique. First, it would not allow me proper air valve control. Second it would not allow me to overblow the bando, etc. Now you may ask yourself why you need to use the air valve much on a unisonoric and I have no answer for that yet as I am still learning, but his recommendation was enough for me to get a Peguri system bando first.
So let me add one more option, as debra noted, most bando players only (or mostly) play on the pull. Which means instead of remembering 4 crazy key mappings you only have to remember 2. So I had the thought that maybe I would take a 142 bisonoric bando and convert it to unisonoric by making the close key mapping the same as the open key mapping. Working on the theory that you can only file a reed up 2 semitones but you can drop a reed much farther (using solder), I immediately rejected this idea. On the key map on the right side there are keys that need to go up +6, +8, +12. I guess you could disable the M reed and drop the H reed and just have one reed on that note. But the left side is worse, there is one key that needs to go down -17 and one key that needs to go up +16. It just looked impossible.
Lo and behold an Argentine bando player recently told me that there is a bando tuner in Argentina that will take your bisonoric bando and turn it into a unisonoric bando by make the close reeds identical to the open reeds. The cost is only 1.5 tunings. Based on what I just said, I have many questions about how this actually works. When I next see the guy that owns such a bando, I'm going to check out the plates to see what was done.
