There are less things to go wrong on a 240D, but there could be more to fix (at first at least) depending on maintenance that the previous owners have done.
Mine got over 30 MPG, and up to 34 on highway road trips...Much better than the 21 I get with a 617 turbo in the same body.
I never had much of a problem with the lack of speed. They are comparable to a VW bus. Never driven a geo, but I expect they are a bit faster.
Depends...if they are in similar condition and you can verify the rebuild was from a reputable place (metric motors) then I would think so.