Notice that Kinesis Advantage supports key remapping and macros so you need a custom firmware only if you need special features like more than two layers or tap keys or whatever. The default Kinesis firmware has two layers (normal and keypad). You can get an easily accessible keypad shift by doing the palm key mod
Indeed - sadly the Kinesis doesn't work with Windows7 at all, so I am on the way to research more whats needed for QMK..
The drop-in replacement I saw looked promising, but its not usable with QMK..
The original Advantage keyboard (KB500) actually works with Windows 7. The problem however is a very specific issue with a USB 3.0 driver on Windows 7 where the keyboard will completely fail. For sometime this problem didn't exist at all until a Windows update that updated the 3.0 driver, which will cause the keyboard to fail. There are a couple of work-arounds - you may be able to change some settings in the BIOS of the computer that will allow the keyboard to work (described under
"USB 3.0 issues" under Tech/Troubleshooting Advantage keyboard), installing a "cable conversion kit" that you can get directly from Kinesis, which essentially is a replacement PS/2 cable and a USB adaptor (installing the PS/2 cable directly to the main circuit board of the keyboard bypasses the usb daughterboard/usb hub, which is the component the win7 usb 3.0 driver has an issue with), and lastly which isn't ideal for many people is to simply upgrade the operating system (the compatibility issue only exists on Windows 7 and does not occur on Windows 8 or Windows 10).
110€ is a pretty good deal, assuming it isn't 10+ years old and in good condition. If you can get the serial number, check with tech@kinesis.com and we can look up the manufacture and sale dates.
The newer Advantage2 keyboard, KB600, has all new electronics built from the ground up and doesn't have any compatibility issues with any operating system. The firmware is much more advanced and will continue to improve. While it does only have two layers like the original KB500, you can certainly achieve four layers a number of ways, one is by using modifier keys to mimic a 3rd and 4th layer (this makes the most sense and is the easiest/simplest solution), and/or by using "hot-key layouts" which the Adv2 supports dozens. You can also program any key on the keyboard (besides the default Progrm & Keypad keys) to behave as a Keypad-shift or a Keypad-toggle, something the original couldn't do (unless you also have an additional footswitch that connects to the back of the keyboard via RJ11 cable, which you could use to remap a keypad-shift (but not a toggle, or maybe I have that reversed)). Btw, the "Keypad" key is our version of an "Fn" key found on many other keyboards, just different terminology mainly due to our Contoured keyboards always having an embedded 10-keypad on the right side (~25 years ago with the Model 100, "fn" key wasn't really a popular term). It could be possible we could add additional layers to our firmware at a later date as well (if it doesn't require a massive fundamental change), and add other useful features. For example, the ability to program different actions for a press and release (lets say holding Ctrl functions as a Ctrl but a quick tap of Ctrl acts as Esc), which is something we are working on (most of programmers time is with the Freestyle Edge at the moment however due to its release).
Currently there is a sale on
refurbished Advantage2 keyboards for $269, marked down from the regular $349, which is a pretty good deal imo since they come with the same 3-year warranty & 60-day return policy as a new. A refurbished model is almost always a keyboard that was ordered online, the customer briefly tried it, decided they didn't like it and sent it back for a refund under our 60-day return policy. We test it, wipe it down with rubbing alcohol and restock it as a refurb. So they are essentially "like new" products with hardly any use. International shipping costs though could be pricey though, depending on where you're at exactly, so that 110€ used keyboard could be a better value for you, but imo if you can spend a bit more money the Adv2 keyboard will be better in the long run and requires far less tweaks to achieve what you're looking for (plus it has mechanical function row keys, which is nice compared to the older rubber function keys).