I didn't even know Introducing the Minutemen existed! I guess that's the idea behind it, isn't it? I still think What Makes a Man is a better start than Double Nickels.
As far as The Mountain Goats goes, when you say you're trying to figure out where to continue from The Sunset Tree and Get Lonely, do you mean those are the only two records you've heard? If that's the case and you like TMG as a band with a fuller sound, you could go for Tallahassee or Heretic Pride. The latter is more or less all about or from the perspective of the same two characters, while Heretic Pride is more diverse. If you want to start getting into TMG's earlier, more sparse stuff, listen to All Hail West Texas.
Edit: beaten to it, but I spent the time typing so I'm gonna post anyway