i dont think a grom is suitable for a luggage rack (due to the extremely tiny dimensions of the bike plus lack of fixtures) unless you want to sit on it in place of the seat.
also the cost of them old ( apprication) or new (over priced toy) is a premium id put as bluntly sunday toy for childish nonsense... also parts wont be as easy to source.
id say a 125, something like we had a honda cbf125, cheap to buy and insure, 100+mpg,
parts new or used cheap readly available, light and easy to ride, luggage options available, simple and easy to fix.
gets up to 60mph easy enough and you can keep with traffic where as its far scarier on a 50cc when you can only do 30 at a push on a flat and everyone baring down or passing you at great speeds, then your a slow tractor on any incline....