Good advice, I would modify it in suggesting a mk25 or Mk11 for first stages, just easier to get parts and service, MK 10 is a good solid reg though.
I'm not fond of the MK11, especially for technical diving where you might want a turret or end port depending on your configuration. I also choose to use MK5/10s rather than MK25s
because of parts availability. To service a MK5/10, you only need the SP seat; three of them come in a service kit and usually two of the three will work for an individual regulator. There are also perfectly acceptable aftermarket seats available. These seats are cheap and readily available. The rest of the kit is simply o-rings that you can get at any supplier, usually fresher and higher quality (if you want) than what comes in OEM kits.
The MK25 seat is really only available from SP or some grey market source, the kits are expensive, and they include bushings that are impossible to find elsewhere. So it's simply more expensive to service MK25s than it is MK5/10s. There's no way around that.
In terms of other parts, really the only one that ever gets replaced occasionally is the piston, either due to damage on the knife edge (which is sometimes repairable) or corrosion on the piston head. I see NOS MK5 and 10 pistons fairly regularly for sale, and it's often cheap enough just to buy another 1st stage. The MK25 piston, while it works really well, has a design flaw; it has an internal o-ring that is officially not user-replaceable. This o-ring will wear out and the piston will leak. The only solution is a new composite piston, which generally costs more at retail than a MK5/10 costs on the used market.
The only rub with using MK5s is the DIN retainer; it's not the universal retainer, and it is hard to find. A friend (long time poster on this forum who no longer is a member) was able to make the universal retainer work in MK5s with a 2mm thick brass washer. I haven't tried it because I found a few MK5 retainers.
Then there is a legitimate issue with IP rise and HP tanks in the MK10. But, if you use a good, fresh, polyurethane 90 duro o-ring on the piston shaft, it holds pretty well, and I have found that the MK11 has an IP curve worse than the MK10, although it goes in the other direction. Try a MK11 sometime at 3500 PSI and check it again at 300, you'll find the IP about 15 PSI higher at 300. It's not good. MK10s tend to rise 7-10PSI from 300 to 3000 (or just a bit above) and then, depending on the o-ring, go up more with HP tanks. I don't know how mch it matters on an actual dive, but it kind of bugs me because I'm a nerd.