As mentioned above, stay with the same brand, and ideally stay with the same models, or at least models that are very similar.
I use the same G250 second stages on everything just to make servicing and field repairs easier. All the first stages are also set at the same IP so that I can mix and match second stages if needed.
I use Mk 17s for the primary gas regs, but I use Mk 10s (or their non swivel turreted Mk 9 version) for stage and deco regs. It adds another parts kit, but the Mk 10 G250 is still a great breathing reg at depth when used as a stage reg with bottom mix, and it is O2 friendly if used as an O2 reg, where the Mk 17 is less than ideal due to the diaphragm design.
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As mentioned by someone above, the Mk 2 is very simple and has no dynamic o-rings exposed to high pressure gas (it still has 2 dynamic o-rings - the piston head and piston stem o-rings - and both of those are in the intermediate pressure section of the reg). As such it makes a great O2 reg - but while it breathes fine at shallow depths (130 ft or less) it's not really well suited to use at depth with a stage. My preference is to use the same basic reg for either stage or deco reg use so that you have some flexibility.
