Does it support the whole length of your body or sag under the heavier areas. Lie there for a while, your body will let you know if the sleep set is right for your needs. Your dealer should be able to fill in the details. If the retail dealer can't tell you how the spring unit works, look elsewhere. This level, the box spring or foundation, is essentially support for the mattress and ideally, to absorb shock.
Some makers use coils, some use modules with modules now becoming the standard.
Little really if the mattress is evenly supported. These support the mattress well, but do little to enhance the comfort of the sleep set. They are cheaper to make and may reduce the cost of the set. If you choose a sleep set with a foundation rather than a true box spring, ask the seller to explain how it is constructed. Some are solid sided and so well made that many institutions specify them for durability.