The reason furnaces and gas water heaters are required to be above the floor in garages is so that “if” you have a gasoline leak in the garage, the fumes, which are denser than air and stay low to the ground, cannot be ignited by any pilot light.

Just how far off the ground each appliance must be and ‘if’ a door is required to separate the garage from the appliances is a matter for your local code enforcement office to decide.

Also, since this is a ‘used’ home, only the codes at the time of the construction would apply, not any modern prevailing code.

For example, if code allowed for these appliances to sit directly on the floor at the time of construction, there is no requirement to move them now or bring them up to modern code.

It would also not matter (usually) if the water heaters were NOT above the floor if they are electric and have no pilot light.

Call your local building code enforcement office for their official opinion as to the ‘legality’ of the current setup. Theirs is the only opinion that matters.