Your question involves the two parts of a bed, the frame and the mattress, which have to be considered separately. An adjustable bed frame is fine as long as it has all the functions of a hospital bed; separate head and foot elevation, total bed height adjustment, about 6 to 8 inches of clearance underneath for a Hoyer type lift unless an overhead lift is going to be used, and the ability to work from both sides of the bed (meaning a full, queen, or king would have to separate into two beds.).
Mattress such as the Tempurpedic, Sleep Number, etc. are for comfort for an able bodied person. They are not constructed with the necessary consideration of preventing pressure sores. They will only be temporary for a person with ALS. These mattresses are constructed using memory foam. Memory foam is great for ALS patients for quite a while in the disease process but not generally for the whole time.
Memory foam overlay pads are the first step in comfort and the prevention of pressure sores. Next there will need to be modification of the memory foam to include layers of medium density and cut out areas of low density memory foam for high pressure areas. This is how medical supply mattresses are constructed and the same thing can be done by purchasing and cutting layers of high, medium, and lower density memory foam.
Next up the hierarchy of mattresses are air flotation mattresses. (There may be gel flotation mattresses but they would be very heavy.) These are not the blow up air mattresses sold as beds for guests! They are constructed of individual but connected air cells so that air is shifted out from under high pressure areas, and are very effective for both comfort and pressure sore prevention. The next level is an alternating pressure air mattress. This step up adds a motor which alternately inflates and deflates sections of the mattress. The ultimate mattress for pressure relief and comfort is a rotating mattress which gradually turns the person from modified side to side position. This can greatly reduce the need to awaken the caregiver for repositioning. As Laurie (lgelb) mentioned, the turning may not be suitable for everyone, especially those with shoulder pain.
In choosing a bed and mattress the speed of the person's progression and money available to upgrade repeatedly has to be considered. Insurance coverage won't apply to Tempurpedic and other non-medical beds. Unfortunately it can be difficult to get insurance to cover medical beds until there is evidence of a pressure sore.