The MK10 shares a seat with the MK5, both among the most successful and popular 1st stages in history. Even if SP were to stop producing rebuild kits for them, which is extremely unlikely, there are aftermarket seats and someone else would undoubtedly get in on the manufacture of them. It's a huge market in terms of scuba parts. Considering you can easily buy these stages for well under $100 on ebay (the cost of a single service for many regs) there is no risk in buying them. The rest of the rebuild kit, other than the seat, is simply a few standard size o-rings. I have several MK5s and several MK10s, and I maintain them well. I have not purchased a SP rebuild kit in at least 5 years. I bought several seats from a dive shop and others online, I probably have 15-20 of them which should last me the rest of my diving career.
The MK15 has a unique seat and was a much less popular reg. That's why SP has stopped supporting them. I have maybe 5-6 seats for those and I might as well use them over the next 10 years. That's why I started using the MK15s as my primary cave set. I also like them very much; I think they may be my favorite of the SP balanced piston 1st stages. They hold IP forever; I've had one go well over 5 years of occasional diving with zero IP creep. But if you're looking at buying a set of SP regs, the MK15 is not the best choice unless you have a good supply of seats, which presumably you don't.
I wouldn't get too picky about the 2nd stages either. The G250, G200B, balanced/adjustable, G500, S600 are all excellent performers, nice and simple (well, the S600 is not so simple) readily available, and economical. They all work fine. I refer the balanced/adjustable for cave diving because it's metal and provides a bit of moisture on those long dives. I also use D series 2nd stages because they breathe extremely well, are totally reliable, and I have at least 15 poppets to keep the several D300s I have in service.
I just wouldn't overthink this process. It's fun to learn about the various models and variants of the older SP regs, but the bottom line is they all work great and are relatively cheap. Regulators more-or-less all do the same thing, so it's not that big a decision.
