Your MK15/ G250 will only need to be O2 cleaned if you are exceeding 40% O2. This is true for all regulators, even new ones. And it's not something that is permanent; as soon as you use it on a tank of regular scuba air (not modified grade "E") it's no longer considered O2 clean. This includes any fill from an unknown source, BTW.
So regardless of the regulator you have, it's likely you'll want to get it O2 clean before using it for high percentage O2 blends. Since those blends are almost always deco gas, it's not a bad idea to have a very simple regulator dedicated for deco use. By the time you're doing dives like that, you're likely spending so much money on other things that getting a simple 1st/2nd stage is the least of your worries. I think a good deco reg would be a MK2 paired with a 109 2nd stage. You can find those used for well under $100, and there's very little plastic in the air pathway to provide fuel for a potential O2 fire. You could probably get them rebuilt and O2 clean for under $100, and use it for five years or longer exclusively for deco before needing another rebuild.
The atomic issue with soaking is that the 2nd stages have a seat saver function that separates the seat from the orifice when un-pressurized. This does remove any wear on the 2nd stage while in storage, but it does also mean that you can't soak the reg without it being pressurized. This would be a deal breaker for me.
There's really no reason for you to replace the MK15. But it's your money, spend it as you wish!
