I personally reccomend scubapro, there are better more expensive regs for sure, but their quality is excellent and they have argueably the best service and parts support of any company out there.
The mk25 will breathe well, but youll want to detune it a bit if its going to get any colder than 40 degrees F, it actually flows a bit much for that design under very cold conditions. (Though we just had reported under 32 degree conditions on the bottom of one of the deeper lakes up here recently, dunno how the hell that happened this early in the year) The second stage is all up to you depending on whether you want to fiddle with all the little adjustments to make breathing easier or are one of those simple is better people.
I was looking at the mk16/550 set, seemed to be best balance of price vs. features and very good for the cold.
Edit: O and as far as whether plastic is a weakness, how often do you need to pound nails with your regulator?
Not more or less than any other reg used in salt water.
