well, i highly vouch for ambient 4. but it ain't about the buxx, it's about the grooves! since you mentioned you love my life in the bush of ghosts and another green world then definitely go with before and after science as it has a little of that david byrne-esque funk sometimes, as well as some of the more hauntingly pretty vocal stuff he's done ('by this river,' linked above).

otherwise, do a preview on youtube if you can't decide. check these tracks from before and after science:
by this river
no one receiving
julie wiith

here come the warm jets:
some of them are old
baby's on fire
on some faraway beach
dead finks don't talk

taking tiger mountain
the fat lady of limbourg
third uncle
the great pretender
mother whale eyeless


the unfortunate long and short of it is that ALL THREE of those albums are essential, so just spend the extra dough and get all three .