Whew, this could be lengthy! But basically, I don't think mods are really necessary much at all. But analyzing each is helpful. I have built or had built for me about six 1911s over the last 20 years, and tried a various combinations of the common mods. I also have some stock guns unmodified. There are others here with much more trigger time than I, and with more guns, and mods, but I have some experience. These answers are going to vary based on whether you are building a production class competition gun, high-end cc gun, or just a want a plinker/nightstand gun.
My thoughts for what they are worth, mostly assuming a non-competition, non daily cc gun are below. When I say "stock" or standard I am referring to what you would find on a typical post-war Colt commercial or Series 70 style 1911.
Trigger: Assuming your gun is reliable and "combat accurate" to begin with, the only mod I've universally found to consistently improve things is a well-polished and installed long trigger, coupled with a flat mainspring housing. This gets you a proper grip and the gun to point more naturally (as it was originally designed before the Army mods in 1923).
Sights: If the gun is for regular target range work, adjustable Bo-Mar style sights make it easier to shoot accurately, but this does not of course make the gun inherently more accurate, or better in a typical SD scenario. Even low GI sights work acceptably well with practice. If going to the expense and time of making new sight cuts (like for a low-mount Novack style), installing non-adjustable sights seems like a waste - they can't be regulated for different loads or distances. Others like them, but I find the lack of versatility for the expense disappointing.
Hammers: I do not find original style spur hammers and grip safety shape to be a hindrance. For LOTS of rounds, an upswept beavertail with a rounded hammer and is more comfortable over time.
Grip Safety: Regular style works fine. For lots of extended shooting, a an upswept type, with a cut-out for the hammer to rest in, is nice. Best to get one with a "speed bump" - otherwise the safety may not deactivate with a high-hand hold.
Slide stops and thumb safeties: Stock type works well. Extended, stipled, tactical, ambidextrous, etc. don't offer that much added benefit. An "extended tactical" (narrow and extends forward along the plunger tube a little bit) seems to be my pick of the aftermarket types.
Stipling, checkering, etc.: Nice but not necessary.
Guide Rods: Add a little smoother, more linear recoil, but no demonstrable improvement in function or reliability.
Shock buffs: Messes with the spring rate, negatively affect reliability. Avoid.
Grips: Smooth or checkered, wood or synthetic, you can learn to like and work with any. I have all types on various guns and it does not make a great difference.
Magazines: Now this does matter. Make sure they reliably work with the rounds you will shoot. I find ones with flat polymer followers to work the most reliably with the greatest range of ammunition.
With all the modded guns, my current favorite is a 70's era Colt Gov't Model, with the only mods being 1) long trigger, 2) flat MSH, 3) group gripper guide rod, 4) Smooth colt medallion faux ivory grips. It is utterly reliable, classic looking, and as much fun to shoot as Custom builds worth three times the price and with all the bells and whistles.
Hope this helps, or serves as a starting point for others to agree/disagree/branch out.
