Moho and ion engines

[Birdco_Space]

In my practical experience (I go to Moho a lot, its a fun place to go), there is a minimum TWR needed at each stage of the voyage to do it for the minimum possible delta-v

for Phase 1 ejection, which is around 2200 m/s of delta -v, if the launch is timed right, a TWR of around .4 to.5 is around the minimum needed to get an accurate ejection to Moho.  going much less makes the voyage there much more expensive in delta-v, by hundred of m/s in correction burns.

for phase 2, this can be done at .1 TWR, though .2 TWR is more efficient.  Ion engines shine here, as you only need a quarter of the solar cell mass per Ion engine, therefore it is easy to build the 2000-2500 m/s delta v stage to capture.  However, you will more than likely need to capture in a retrograde orbit (not a big deal given Moho's relatively slow rotation

Phase 3, landing, you need at least a TWR of .55 to do it easily, lending itself to a stage with around 1200 m/s delta v for just a landing, 2500 for return to orbit as well, with a small vacuum optimized LF O engine.

Ions work for return and Kerbin Capture, but you'll need about 4x the mass of solar sells, or a sizable mass budget of batteries for the kerbin capture. 

Breaking this out:  A 1.25 or 2.5m vacuum LF O engine for kerbin ejection.  A Ion stage for moho capture, and small LF O stage for Moho landing.  Round trip requires a sizeable Ion stage, and docking in Moho orbit. 

My unmanned surface probes to Moho are launch able with 1.25m parts and asparagus staging, or more simple 2.5 meter rockets.