I took a more detailed look at what you're proposing for logistics.
The reason a group buy is run is usually to get prices as low as possible. If you want to artificially limit the number of sets to ~50, you run the risk of having a double problem: You have more people who want to get into the buy than you have sets and problems are caused, and you miss out on potential lower prices for the community due to economies of scale at higher quantity price breaks. If you take a look at some of the other keyset buys run through SP, many of them have price tiers for different kits, and quite often the savings moving from 50 to 75 sets is significant.
Another thing that comes to mind is if you want all your caps to be mono-legend, that could require new legends to be cut. I don't know how many of those legends would be available, but worst-case scenario is you're paying for 20 new legends to be cut (21 if SP doesn't have "escape", I know they have "esc" though). Legends, IIRC, cost $40 or so per key, so that could be around $850 in up-front costs that you'd have to either eat yourself, or spread across the cost of the sets -- increasing the average price of each set by $17, if you do only 50 sets. Here's another argument for opening the set up to as many buyers as possible.
Now, I'm not trying to discourage you from running the set, but I would suggest that you do some more planning before you move forward planning on a small number of sets. Once you know if there is enough interest in the concept to make more effort worthwhile, I would get in touch with SP. You'll want to ask about any legends you want to use that aren't 100% standard, and see if you can figure out what exists, what doesn't, what you can make (and what you maybe can't), and the process to make new legends, if necessary. Note also that available legends will vary by the key profile you choose. Keep price in mind as well here -- every new legend you make adds a cost to the set, every key you add adds a cost to the set, and if the price goes too high, or if there aren't enough keys to cover common layouts, you'll have a very hard time getting orders.
There have been successful group buys for 60% only sets before (Nostalgia set), but that set was compatible with much more than just an ANSI 125 board, which is what you've mocked up. If you want to be successful, you need to strike the balance between appealing to a wide enough audience to get prices low enough that everyone can justify the purchase, and including few enough keys that the set does not spiral out of control.
Most of this has been largely stream-of-thought, but the point I'm trying to get across is that it can be complicated to design a key set, as much if not more so than handling the shipping and distribution.
