Whilst not nullifying your comments, three pax can be as distracting as five - and 150kts is likely to be only 20-30kts faster than the four seat they might otherwise buy. Your 180kts example is a considerably more significant (50%) increase on 120kts...
You should also be careful about buying something you might 'grow out of' very quickly.
Buy the higher performance aircraft, and invest in a 'bring along instructor' (or indeed any experienced pilot) for the first few long trips. I guarantee they'll charge little to nothing if it's a fun trip...