I bought a MK I target new back in the late 70s. It looks just like the one you have except for the walnut grips. The bull barrel adds weight and stability to the pistol. I can hold mine noticeably steadier than the much lighter SR9c. When I used to shoot at an outdoor range, I would set up spent shotgun shells on the 25 yd berm and use them as targets. Great fun.
Maybe I am not a "connoisseur", but I can't imagine spending the time and money on a trigger upgrade to this weapon. The factory trigger is extremely good. Short, smooth, crisp and light. I don't think I've ever had the opportunity to shoot the standard trigger.
Never done anything to mine in the last 35+ years other than clean it.