These are the core add-ons for the stock game that add more realism, but don't mess with the tech tree or add a ton of parts. The core game plus it's expansions have a boatload of parts now, and I find most everything I need in it.
Certain addons are not currently available on Spacedock, but can be downloaded via the CKAN app:
Kopernicus can be downloaded from it's GitHub page:
https://github.com/prestja/kopernicus/releases
Outer Planets Mod can be downloaded from it's GitHub page:
https://github.com/Poodmund/Outer-Planets-Mod
Stock Visual Enhancements can be downloaded from it's GitHub page:
https://github.com/Galileo88/StockVisualEnhancements/releases
Stock Visual Terrain can be downloaded from it's GitHub page:
https://github.com/Galileo88/Stock-Visual-Terrain/releases
USI Life Support can be downloaded from it's GitHub page:
https://github.com/UmbraSpaceIndustries/USI-LS/wiki
Enjoy!