Welcome! For going to Duna you might want a larger variety of parts, I really like @CobaltWolf's Bluedog Design Bureau and @Beale's Tantares and TantaresLV. They both add a LOT of stuff to the game, from probe cores to fuel tanks (and everything in between). They also have a few lightweight probe-related things, which I like so I can use Agena to send a probe out to Eeloo (never done that, but it's easily possible).