a. Yes sand will be fine for the guppys and plec, in fact they'll enjoy it more than gravel because it doesn't trap so much waste in it as gravel can.
b. You can clean the sand by simply stirring it up thoroughly with your hands in the tank and then doing a water change, so you don't really need a sand/gravel vac. You will need to upgrade your heating and filtration though and run the new filter alongside the old one for at least week with the fish in the new tank.