For interior decorating preparation (keying gloss paintwork, removing loose
paint, rubbing down emulsion-painted walls, etc.), what type of electric
sander is best?

Should I consider orbital or random orbital? Palm size, one-third sheet or
half sheet? Or a circular abrasive sheet?

I would hope to find something suitable that did not vibrate too much, and
which would let the dust be sucked up a vacuum cleaner attachment. Is it an
advantage to have a sander with variable speed?