I can’t be positive that first second-stage I tried was a 109, but it sure breathed easy. I like the “idea” of using vintage equipment, and I already have this. I hear nothing but praise about SP and their parts/service life. After reading these replies, I’m leaning more towards the option of offloading the Mk1 and getting a Mk5 or Mk10. With the parts availability, it seems like an interesting project.
It always saddens me that dive shops would call 109's old trash, but still be willing to sell you a r195, which is basically a 108 ( a less sophisticated reg) in plastic with a venturi ... It says more about the dive shop owner's knowledge of regulators than anything else. It is true that modern regulators have bigger exhaust valves, which is nice for exhalation resistance, but I've yet to meet a person who had 'issues' with the exhalation resistance of any 108,109 or balanced adjustable.
The 109 has been officially paired with the mk10 in the past, that's why I'd suggest getting that one, to increase the chances a dive shop would be willing to service it. The mk10 is also the oldest regulator that's still officially supported by Scubapro if I remember correctly, so parts and kits will be on hand.