The only mod I have installed is NovaPunch. I know there are some problems with it, but I enjoy playing around with those parts... (and from what I've read, it also makes me ineligible for the challenges )
Strangely enough, I ended up putting my first non-Kerbal system satellite around Duna after missing an approach on Minmus and deciding to make the best of a bad situation. It was definitely a learning experience.
Got my first satellites around Moho and Jool and learned I could use monopropellant in a pinch as full scale propellant to decelerate a ship. Made all the difference between being 81m/s too fast to achieve Moho orbit and a long, lonely solar orbit for a mission I worked diligently to complete.