Smooth trigger? Absolutely - if it makes that much difference to you. I don't notice the trigger feel difference between my G17 and G26. If you are going with a "-" connector, replace these parts together.
Sights? I'm a sucker for OEM steel white outline rear and tritium front. I put them on my Gen1 G17 and have never been happier.
Frame plug? Do it. Easily removed if you don't like it.
"-" connector? Only if OEM available, and I recommend replacing trigger/trigger bar at the same time.
Lasermax guide rod? Not for me. I don't like lasers in general except for SWAT shield work. And, I'm of the school of thought that Glock pistols tend to work best with OEM Glock parts.
Refinish? Why bother. It's a Glock. Remember that the black surface finish is cosmetic. Tennifer is IN the metal, and still protecting even if the black phosphate wears off. Gen2 Glock slide finishes tend to get "doggy" over time, but it doesn't affect hardness/corrosion resistance.
Glock 17, 19, 21SF, 26, 37, 39
Glock Advanced Armorer (AA), Glock Instructor Workshop (IW)
Remington 870, Benelli M1, Smith & Wesson 3000, Mossberg 500