You will need the Tern rear mech adapter before you can do anything to the rear cassette..I think the standard derailleur will only work with a 30 tooth cassette..It might be easier and cheaper to change the chainwheel to a 42 or 44 ..That would give you a low gear and a reasonable top gear..
Bar ends are easy enough to fit..Use small ones though because they may cause trouble with the fold..
Dahon make a pannier that should fit your bike..I would think Tern do also..A pannier rack bag may be better because you may catch any panniers with your heels..A front pannier rack from Dahon may fit..Do not bend any alloy arms on the rack..alloy /aluminium does not like being bent!
You should find one ..Perhaps looks for one to fit a 24 inch wheel bike and then you may be able to fit full size panniers..