If you're going interstellar, nuclear pulse drives are the only way to go, so I suggest roverdude's new orion drive, also roverdude's other base building mods. There's also something called pathfinder. You could use a mod's resource system to make it harder.
One thing you could do is add no parts, but have a list of suggested mods to make it possible to reach other stars, such as roverdude's orion drive, maybe those OPT spaceplane parts.
Thanks, however I have changed my google account name. Nothing personal, I just don't want anyone to be able to see it, especially spambots or stuff like that. Or maybe it didn't work, google accounts seem to be absurdly strange.
I made a mod Gemini a long time ago, I'll see if I can find it. - - - Updated - - - Here we go: http://forum.kerbalspaceprogram.com/threads/113029-Gemini-Titan?h I forgot how terrible it was, please don't judge my later work by it.