Choosing a box spring

[inquisitive]

I am thinking of buying a box spring separately from a mattress in order to save money. As far as I can tell, the purpose of a box spring is twofold: 1) to add height. 2) to provide a flat surface for the mattress to lie on in order to support and preserve the mattress. Does quality vary among boxsprings and what do I look for? I was thinking of buying this one due to the storage space underneath:

Amazon.com: Spirit Sleep 14-Inch Better Than a Boxspring with Coverlet Queen : Home & Kitchen

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It seems to double as a bed frame + box spring. How can I tell if it is strong enough/provides a flat enough surface? I may buy an expensive mattress like a tempurpedic.