Late game starts basically when you have a strong survivor with decent armor, a big pile of food and meds and either a safe base or a deathmobile. From there, you can do pretty much whatever you want, since cdda is a sandbox roguelike, there are no fixed objectives. Clear cities of the undead, build a new city for your followers, or basically anything else you want.
that’s 2 questions! 
There is no ‘decent set-up’ to reach lategame. Every setup works, and it depends on your playstyle. If you want to minmax stuff, go for a strength build and preferably melee weapons or bows, since those are silent and it’s easier to get more melee weapons or arrows in case stuff breaks.
Construction Scaling is a percentage the game multiplies to the time necessary to build stuff. For example, if your normal build time is 1 hour, putting the construction scaling to 200 raises the build time to 2 hours. If it’s 50 it halves the time necessary to build something. Not sure why you’d ask this, it’s explained in the world options.
Depends, Deathmobile or stationary Home? Ants are an easy way to get meat. Fishing can be quite good as well, as long as you do it at a river. Bee hives work well too and can give you honeycombs and royal jelly, which is useful. If you have a stationary base somewhere, put a farm there, for potatoes and pumpkins - those are, in my experience, reasonably good. You’d also want to grow Wheat and tomatoes, for bread and fancier recipes.
