So, I should probably say where I got in KSP so far (haven't played in a couple days, though)...
I've got a station with four kerbals inside, two T800s for fuel storage, a crapton of RCS, and the ability to dock one ship (the interplanetary ship in this case) on the end, and a resupply ship I've designed on the side. I am planning on launching another fuel storage section sometime, but holy crap that was a scary stage to get into orbit, it BARELY made it up to rendezvous, it was a huge pain to control, and... yeah. I might try burning on the pad to jettison some weight, and use resupply ships (which are a stable design) solely to supply fuel, but it'll still be ugly.
All of my ships lately are actually based on a Mun ship I designed, and it turns out that with application-specific tweaks (the top stack changed and some fuel added for interplanetary, things slapped on top of the capsule for placing station parts into orbit, and a bunch more fuel and RCS thrusters added for resupply ships), it works amazingly well.
I've done an attempted Duna mission, and actually landed on Duna, but due to a ship design issue, a staging issue, and insufficient fuel, I've got a Kerbal stuck on Duna. (Surprised the Kerbal survived the crash landing (parachutes don't work well on Duna) after the failed attempt at launching from Duna.) I've got an upgraded ship design for a somewhat Apollo-like 2-stage ship, to drastically reduce fuel demands for deorbiting and landing (fuel demands for takeoff will be the same, but the key here is to have the fuel to begin with).