Horses need a lot of forage, about 2-3% of their body weight per day, so for a 500kg horse you're looking to feed 10-15kg of forage per day, forage can be grass, hay or haylage.
Weather or not the horse needs additional calories will decide what, if any, bucket feed you need to give, good mixed species grass will provide everything they need generally, but if part or all of their diet is made up of preserved forage (hay/haylage) you will likely need to supplement certain vitamins and minerals that are depleted in the preserving process.
Balancers contain vitamins, minerals and proteins without too many calories, so are a good option for an easy keeper, vitamin and mineral supplements are also an option, they are similar to balancers but don't contain protein. Complete feeds are another option but they will contain significant calories, so are more suited to hard keepers.
Personally I try to avoid adding to many grains to my horses diets, most of mine don't need the extra calories or energy, the one who does need more calories gets a bit more excitable on it and ends up running the extra calories off in the field so it doesn't work as well for him. I like fiber based feeds that are digested more slowly and release their energy more slowly.
There is no simple answer to what feed is best, it really depends on the horse you get, where you keep it (how much turnout they get, how much and quality of grass there is) and what you do with the horse so its really impossible to decide before you get your horse.